Print engine for color electrophotography

ABSTRACT

An improved full color electrophotographic print engine using flexible belts to carry a photosensitive electrostatic image developer and a transfer belt for building up composite developed images. Each belt, and an optical scanner, are driven by mechanically independent motors which are synchronized by an electronic digital controller implementing precise servos. The use of the flexible belts allows the machine to be relatively inexpensive, very small compared to previous full color print engines, and yet maintain precise registration of color composite images. An improved fuser mechanism with increased dwell time at constant machine speed is also shown. The fuser includes a pair of spaced-apart rollers, both of which urge a sheet of image receptor against the heated roller over a predetermined angular portion of the roller. The machine is specifically designed to be used interchangeably with an optical bench source as a copier or with a laser bench as a laser printer. Improved copy quality monitoring by sensing actual amounts of toner deposited on a photoreceptor belt is also shown. Also, improved downwardly facing gravity-fed toner deposition modules are shown including an embodiment which has a purely magnetic gate for opening and closing the toner supply.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 791,218 filedOct. 25, 1985 now U.S. Pat. No. 4,652,115, entitled "IMPROVED PRINTENGINE FOR COLOR ELECTROPHOTOGRAPHY".

TECHNICAL FIELD

The present invention relates to print engines for colorelectrophotography and, in particular, is an improved mechanism having aplurality of individual mechanical subsystems which are drivenindependently by an electronic controller which synchronizes operationof the independent subsystems. The present invention also includes animproved fuser and print color monitoring systems.

BACKGROUND OF THE INVENTION

Electrophotography has become one of the most widely used systems in thefield of information processing in this century. In particular, drycopying, also known as xerography, has become a standard process forcreating copies of documents in a host of environments includingoffices, educational institutions and the like. The basics of xerographyare well known to those skilled in the art.

The fundamental elements of a xerographic copier include aphotosensitive medium which is charged with an electrostatic charge ofpredetermined polarity. An optical image of an original to be copied isfocused onto the electrostatic medium, either at one time through theuse of a stroboscopic flash and appropriate optics, or by a linear scanwhich moves either the light source, optical elements in the opticalpath, or both, in synchronism to scan the photosensitive medium with theimage of the original.

Portions of the originally uniform electrostatic charge on the surfaceof the photoreceptor are conducted away from the areas of thephotoreceptor which were illuminated, and thus an electrostatic image ofthe original is left on the photoreceptor. In most modern xerographiccopying systems, this image is passed over a source of toner materialselectrostatically held to ferromagnetic carriers. The ferromagneticcarriers are used to allow magnetic fields to bring the materials intocontact with the above-mentioned electrostatic image.

The electrostatic charge which remains on portions of the electrostaticimage has a sufficiently strong electrostatic force to pull the tonermaterials away from the carriers and to hold them in place on theappropriate portions of the electrostatic image. The magnetic forcesassociated with the toner modules carry the ferromagnetic carrierparticles back to a position where they are remixed with additionaltoner.

As is known to those skilled in the art, the toner materials arenormally plastics which melts at a predetermined temperature and haveappropriate color characteristics once they are melted.

The charged photoreceptor which now carries toner on the portion of thephotoreceptor which was not discharged in response to light when theelectrostatic image was originally created, is referred to herein as adeveloped image. Subsequently, the photoreceptor carrying the developedimage is brought into contact with an image receptor which, in the mostcommon applications of xerography, is a sheet of paper. Electrostaticcharging techniques are used to transfer the toner from thephotoreceptor to the image receptor.

Once this is accomplished, the image receptor is passed through adevice, commonly referred to as a fuser, which is a station in the pathfor the image receptor at which the transferred toner is heated to fixthe image onto the image receptor. By this process, a monochrome copy ofthe original image which was exposed onto the surface of thephotoreceptor is made.

In more recent years, systems for color electrophotography have beencreated. In many respects, the process of color electrophotography isanalogous to standard three-color printing processes used in the moreconventional printing arts. Conventional three-color printing colorcomponent images, commonly referred to as color separations, are createdby photographing the original through appropriate filters. Each of theseparations is in turn made into a separate printing plate. During theprinting process, each plate is inked with an appropriate colordetermined by the filter used in making the original separation. Theprinting press can be adjusted so that proper registration, aligningeach separate color component image over the other is accomplished. Oncethe press is properly adjusted, multiple copies of the original colorimage may be faithfully reproduced.

Prior art color electrophotography machines have used a conceptuallysimilar process. Most full color dry copiers use three process colors torecreate (within the color limits of available toner materials) anycolor in the spectrum. Three color component images are shot throughthree separate filters in a manner analogous to the creation of colorseparations in color printing. Each image is developed with a tonerhaving the appropriate color characteristics, and each developed colorcomponent image is in turn transferred to the image receptor or paper,one on top of the other, to provide a composite image. The papercarrying the composite image is then passed through a fuser in aconventional manner.

It is known in the art of color xerography to include an intermediatetransfer medium between the above-described photoreceptor, upon whicheach individual color component image is developed, and the ultimateimage receptor or paper. In this specification, such an intermediatetransfer medium is referred to as a transfer medium. In this type ofdevice, a composite developed image is built up, one color componentimage at a time, until an overlaid composite color image, havingportions of all three of the color component toners thereon, is createdon the transfer medium. Once this is accomplished, the composite imageon the transfer medium is transferred to the paper which then passesthrough the fuser in the normal fashion.

As noted above, color xerography is conceptually quite similar toconventional color printing. However, there is a significant differencein the economics of scale. Most importantly, color printing is rarelyundertaken for small numbers of copies. In practical environments, thecolor printer normally has ample opportunity to make sure that elementsof the press are properly aligned so that proper registration isobtained. In the absence of proper registration, the individual colorcomponent images are misaligned and the result is a fuzzy image, withedges of objects being outlined inappropriately with portions of one ofthe color components. In color xerography, there is no ability by theuser (nor the time) to make precise adjustments, since often one or twocopies are all that are being made at any one time.

Therefore, in prior art color photocopying machines, the mechanicalelements carrying the photoreceptor medium, the intermediate transfermedium (if used) and the paper have had to be machined to extremelyclose tolerances in order to maintain proper registration. In the priorart, this has only been practical by using relatively large drums tocarry the photoreceptor and critically machined, and thereforeexpensive, gearing arrangements by which the entire mechanism is drivenfrom a common prime mover. Naturally, as these mechanical components ageand the mechanical elements controlling registration suffer wear,registration, and therefore copy quality can, and does suffersignificantly. Therefore, maintenance of critical mechanical alignmentsin prior art full color electrophotographic systems have been one of theprincipal factors in keeping the costs of such machines very high withrespect to the cost of monochrome copiers.

Additionally, it is well known to those skilled in the art that lightsources of considerable intensity are needed in full colorelectrophotographic printing since, during the shooting of each colorcomponent image, a significant fraction of light from a white lightsource is blocked by the separation filters interposed in the opticalpath between the original and the photoreceptor. This, together with thepower requirements of the conventional fuser mechanism and the electricmotors to operate the above-referenced large mechanical drums, created astate of the prior art in which no full color copier known to theinventors of the present invention has ever been designed which canoperate reliably from a conventional fifteen or twenty ampere 120 voltbranch circuit.

Also, because of the use of finely machined parts and high powerrequirements, the prior art has heretofore not produced a practical fullcolor copier which is of a size substantially equivalent to conventionaltable top monochrome copiers.

As is also known to those skilled in the art, conventional full colorphotocopying machines will output copies at approximately 1/3 the rateof an equivalent monochrome machine since three separate images must bedeveloped for each copy. Additionally, all prior art full color copiersknown to the inventors of the present invention have copied monochrome(black and white) originals by use of the standard color copyingprocess. This is known in the art as creating a copy that is "processblack". This refers to the fact that in conventional full color copiers,three separate substantially identical images are created when themachine is copying a black and white original. Each of these images isdeveloped with a separate color component toner and the composite imageis fused. If the toner color characteristcs are right, and registrationwithin the machine is adjusted properly, the resultant copy willapproximate the black and white original.

However, it is well known to those skilled in the art that process blackfrom color copying machines does not produce as sharp an image asconventional black and white copies made on monochrome xerographicmachines using a single color black toner. Additionally, because threeseparate images are shot, even to copy a black and white original, thecopy output rate for conventional full color copies has been too slow toencourage the user to make use of such a machine as a general purposeoffice copier.

The combination of the slower copying rate (due to creation of threeseparate images) and the lower copy quality of process black monochromecopies has made prior art full color copiers impractical for use asgeneral purposes office copying machines. Therefore, prior art fullcolor copiers have been limited to a specialized set of applications forwhich full color copies are needed on a regular basis, and the volume ofsuch copies will justify the high cost of a full color copying machine.Virtually all such applications for prior art full color copyingmachines have been in environments which require the user to purchase orlease an additional monochrome copier to do more conventional black andwhite copying.

It is also known to those skilled in the art for any given tonermaterial, or set of toner materials in the case of a color copier, acertain amount of heat per unit area must be transferred to the imagereceptor or paper carrying the toner in order to properly melt theplastic and fuse the image on the copy. The heat transfer is determinedby a combination of the temperature on a rotating surface of the fuserwhich contacts the toner material, and the dwell time of the copy in thefuser. The dwell time is the time any given point on a particular copyis in contact with a heated surface within the fuser.

Conventional fusers are normally formed from a roller having a heatedcompressible surface and a compression roller which is urged against theheated compressible surface. The paper bearing the developed image oftoner is passed between the heated roller and the compression roller.Since the compression roller deforms the compressible heated surface tosome degree, there is a predetermined length along the path of travel ofthe paper for which the paper is in contact with the compressiblesurface of the heated roller. Therefore, dwell time may also beexpressed as the product of the path length along the compressiblesurface of the heated roller for which the compression roller causes thecopy to contact the compressible surface, times the linear rate oftravel of the copy through the fuser mechanism.

As is known to those skilled in the art, the process of fusing adeveloped toner image onto the paper, or other image receptor, is oftenthe rate limiting step in the copying process which limits the totalcopy per unit time output of a copying machine. As toners requiringgreater amounts of heat for fusing become used, the only choicesavailable to the designer are to increase the distance along thecompressible surface of a heated roller for which the image receptor isin contact with the surface, to elevate the surface temperature of theheated roller in the fuser, or to slow down the rate of travel of thepaper through the fuser.

Naturally, there is a practical upper limit to the temperature which canbe maintained on the heated surface of a fuser in order to prevent theimage receptor from catching fire or being scorched (in the case ofpaper) or melting (in the case of plastic transparencies), and alsothere is an upper temperature limit at which toner materials will havetheir color characteristics altered.

In conventional color copiers, all mechanical drive devices have beenoperated by a common prime mover in a manner which was mechanicallysynchronized. Therefore, the rate of travel of the image receptorbearing a developed full color image through the fuser mechanism hasbeen substantially equal to the linear rate of movement of the imagebearing surfaces within the machine. For example, the rate of travel ofpaper through the fuser in a conventional color copier has been equal tothe rate of travel of the photoreceptor bearing the electrostatic imagepast the source of toner material. This has led to a trade-off in thedesign of such machines which requires that either the fuser temperaturebe elevated, thus increasing the power requirements of the machine, orthat the entire operating speed of the machine be slowed down in orderto provide sufficient dwell time for the copy within the fuser.

Prior art color electrophotographic print engines have had tonerdeposition modules which faced upward. In other words, the path from thetoner and carrier reservoir to a decorator roll, which eventuallycarries the toner to the photoreceptor belt, was pointed upwardly,either vertically or in a direction with a predominant verticalcomponent. It is believed that the conventional wisdom of those skilledin color electrophotography indicated that a plurality of color processtoners could not be arranged in downwardly facing toner depositionmodules in a practical machine.

One problem which exists in prior art color electrophotographic printengines is cross contamination of the color toners when a phenomenonknown as carrier pull occurs. Carrier pull results from a condition ofunusually high static charge on the photoreceptor. This normally occursin response to a malfunction of one of the coronas which applies chargeto the photoreceptor. The phenomenon of carrier pull is characterized byvery strong electrostatic force between the highly charged portion ofthe photoreceptor and the toner particles on the portion of a decoratorroller under the photoreceptor. These forces are so strong, that theypull large clumps of toner and its carrier particles off the decoratorrollers onto the highly charged portion of the photoreceptor. As thephotoreceptor continues to move, mechanical vibration can shake theparticles loose where they will fall onto the decorator roller of adownstream toner module of another color. Note that this results, inlarge part, because the toner deposition modules are upwardly facing andthe force of gravity tends to pull the excessive toner off thephotoreceptor down onto the decorator rollers which become contaminated.

Also, when the phenomenon of carrier pull occurs, the magnetic particleswhich have been pulled off onto the photoreceptor tend to be attractedto the magnets of downstream decorator rolls. The combined influence ofgravity and the magnets of the downstream decorator rolls which pull thecarrier particles, and thus some of the color toner particles in theimmediate vicinity, onto the other decorator rolls is another mechanismwhich causes contamination of other toner modules in the presence ofcarrier pull.

As is also known to those skilled in the art, the basic technologyapplied to xerographic copying has been applied more recently to devicesknown as laser printers. Conceptually, the print engine in a laserprinter is substantially identical to the print engine used inxerographic copying machines. The fundamental difference between the twois the source of the image to be printed. In the case of a copier, anoriginal is illuminated by a high-intensity light and the image from theilluminated original is focused onto the photoreceptor. In the case of alaser printer, control circuitry is used to turn a laser beam on and offas it sweeps a raster pattern over the surface of the photoreceptor todirectly create the image to be reproduced. As the complexity of imagesto be created by the laser printer increases, and the pixel resolutionincreases, the computational power and memory requirements for circuitryassociated with controlling the laser beam also increase. However, oncethe above-referenced electrostatic image is created on thephotoreceptor, the principle of operation of a laser printer and anxerographic copier are identical. Color laser printing is accomplishedin a manner analogous to color copying by computing a color value foreach pixel and a relative intensity of the laser beam for each colorcomponent of the pixel color and creating three separate images, asdescribed hereinabove.

The foregoing description is, to the best of the knowledge of theinventors of the present invention, an accurate description of the stateof the art for color electrophotography prior to the invention of thepresent invention. From the foregoing it will be appreciated that theprior art has not produced a mechanism for a full color print enginewhich may be used in either a photocopying machine or a laser printer,which may be practically housed in an enclosure which is substantiallythe same size as conventional table-top convenience monochrome copiers.Furthermore, the prior art has not produced a full color print enginewhich will maintain the critical registration necessary to produceaccurate full color copies by overlaying separate color component imageswithout the use of relatively expensive finely machined mechanicalparts. Additionally, the prior art has failed to produce a full colorphotocopying machine which will practically serve as a standard copierin most office environments by producing monochrome copies at either anadequate rate, or with acceptable copy quality for high volumemonochrome copying. The latter limitation has been due to thesubstandard quality of process black as compared with conventionalmonochrome copiers using black toner. Also, the prior art has beenunable to produce a practical full color photocopying machine for whichthe power consumption of the machine is controlled so that it may beoperated from a conventional 120 volt 15 or 20 amp branch circuit.

SUMMARY OF THE PRESENT INVENTION

The present invention overcomes the above stated technical problems inthe prior art in a number of significant aspects. Broadly stated, thepresent invention provides a full color print engine for anelectrophotographic process which includes a conventional photoreceptorfor developing color component images and an intermediate transfermedium for developing a composite image. Broadly stated, the presentinvention implements these two elements with non-critically machinedsurfaces, and employs a novel electronic control arrangement tosynchronize the relative movements of these mechanical parts.

In the preferred form of the present invention, the photoreceptor andthe intermediate transfer medium are both carried on flexible belts.Each belt is independently driven by a direct current electric motor.Light chopper motion transducers are used to detect and quantifymovement of the belts and the passage of an index mark on each belt pasta predetermined reference point is also detected. The present inventionemploys a precision digital electronic controller for synchronizing themechanical movements of the two belts to assure proper registration ofthe color component images when the composite image is formed.

It is an important aspect of the preferred form of the present inventionthat the controller not only synchronize the movements of these twobelts, but also that it continuously adjusts for slight deviations inthe lengths of the belts from their optimum lengths in order to maintainproper registration. In the present invention, the length of one of thebelts is nominally an integer submultiple of the length of the other. Inthe preferred embodiment, the belt carrying the photoreceptor isnormally twice the length of the intermediate transfer belt. However, itshould be understood that as used in the specification, the concept ofan integer submultiple includes a value of one wherein the belts arenominally of the same length.

In the preferred form of the present invention, an additional novelmechanism is used for maintaining proper registration between thephotoreceptor belt and the intermediate transfer belt. In a preferredembodiment of the present invention, the photoreceptor belt rotatesabout two rollers with relatively small radii compared to the overallbelt length. The intermediate transfer belt is positioned to partiallywrap around one of these rollers at the point where the two beltscontact. This provides a surface area, preferably in excess of one-halfinch along the direction of travel of the belts, at which the two beltsare in intimate contact while the color component images from thephotoreceptor are being transferred to the intermediate belt. By properselection of surface charge density and polarity, very strongelectrostatic attractive forces are created between the two belts whichaids in maintaining proper registration during the image transfer.

According to another aspect of the present invention, anelectrophotographic print engine is created which is usable in axerographic copying machine which can function as both a general purposemonochrome office copier and a fully color copier. The present inventionovercomes the above-cited drawbacks of the prior art by employing afourth toner which is a conventional black monochrome toner. A userselectable input to select monochrome copying is provided. Therefore,the present invention will provide high quality black and white copieswithout the drawbacks of copies made using process black, describedabove.

In addition to the higher quality monochrome copies obtained using adedicated black toner rather than process black, the controller for thepresent invention shoots only the single required image when makingmonochrome copies. Therefore, the present invention has a copy outputrate for monochrome copies which is approximately three times greaterthan its output rate for color copies, unlike conventional color copierswhich use process black for copying monochrome originals. According tothis aspect of the present invention, the single developed image usingthe black toner is immediately transferred to the intermediate transferbelt and onto the paper when the device is in a monochrome mode ofoperation.

In the most preferred form of the present invention, a fifth tonermodule is also used which was designed to carry a color specific tonerfor a custom color which cannot be adequately reproduced using the threecolor process toners.

Another novel and advantageous aspect of the present invention is theemployment of independent drive mechanisms for the fuser and the imageformation elements. In the preferred embodiment, the paper path includesa space, between the station at which a composite image from theintermediate transfer belt is transferred onto to the paper and thefuser, which is of sufficient length to hold a sheet of the mostcommonly used length of paper. Therefore, a complete composite image istransferred on to the sheet of paper carrying the copy before theforward end of the sheet enters the fuser. In the preferred form, thepaper carrying the developed composite image is driven through the fuserat a slower rate than the machine rate at which developed images arebeing transferred from the photoreceptor to the intermediate transferbelt. Since, during full color copying, three separate images must beshot and developed before composite image is transferred to a sheet ofpaper, the speed through the fuser may be lowered to as much as 1/3 ofthe above-referenced machine speed in embodiments of the presentinvention. This provides an advantageous mechanism for increasing thedwell time of the copy in the fuser without slowing down the overallcopying rate.

According to yet another aspect of the present invention, the controllerfor a print engine embodying the present invention controls the maximumpower drawn by the machine during operation. This allows embodiments ofthe present invention to be constructed which may be dependably operatedfrom a conventional 15 amp 120 volt branch circuit so that the user ofthe present invention does not have to suffer the expense of installingspecial branch circuits in the environment in which the print engine isused.

According to another aspect of the present invention, an improved fuseris provided which increases the dwell time of the image receptor in thefuser mechanism without slowing down operation of the fuser orincreasing the fuser temperature. The improved fuser of the presentinvention includes at least two compression rollers for urging the imagereceptor against the compressible heated surface of the fuser and thuseffectively doubles the dwell time without affecting other operatingparameters. The preferred form of the improved fuser includes means forurging the copy against the heated roller of the fuser so that itdependably feeds the paper from the position where it exits the stationat which the first compression roller and the heated roller are urgedtogether and enters the station at which the second compression rollerand the heated roller make contact.

Yet another improvement of the present invention is an improved copyquality monitoring system. As is known to those skilled in the art, anumber of environmental variables affect the operation ofelectrophotographic print engines, particularly humidity andtemperature. In the presence of a relatively high humidity, the airincludes a lot of hydrated ion carriers which can have the effect ofdischarging the electrostatic potential applied to the photoreceptorbelt. This in turn affects the amount of toner transferred to thephotoreceptor during the development process. Similar effects will beobserved in the transfer from the photoreceptor belt to the intermediatetransfer belt.

The print quality monitoring system of the present invention includestwo major subsystems: an electrostatic monitoring and toner depositionmonitoring. The first element of the system is provision of a referencestandard for creating a reference image segment on the photoreceptorbelt. In embodiments of the present invention used in photocopiers, aset of standard color component bars are provided slightly to one sideof the platen glass for holding originals to be copied, and on theinterior of the machine. These are aligned within the optical path ofthe optics of the system so that when a scan of an original is made tocreate an electrostatic image on the photoreceptor belt, the color barsmay also be selectively scanned to form an image on the belt. Anon-contact electrostatic volt meter is disposed over the photoreceptorbelt. The controller for the present invention keeps track of movementof the portion of the belt carrying the image of the color referencebar. When the image of this bar passes under the non-contactelectrostatic volt meter, a reading of the volt meter is made.

The electrostatic voltage on this portion of the belt is a knowncalibration quantity. If the electrostatic volt meter detects a voltageon this portion of the belt which is outside an acceptable range, aprocessor within the controller addresses a look-up table and determinesan appropriate change to be made in the input voltage to the coronacharging the photoreceptor belt. In this manner, environmental effectswhich may alter the electrostatic characteristics of the photoreceptorbelt are offset.

The second subsystem of the print quality control apparatus is a systemfor monitoring the depth of toner deposition. When print qualitymonitoring is taking place, the controller again tracks movement of theportion of the photoreceptor belt carrying the electrostatic image. Thisimage is developed as the belt passes the active toner station (whichcarries the color of toner corresponding to the color of the colorreference bar used in creating the image) and is developed in a normalmanner. The developed reference image then passes over an opticalscanner, the output of which is read as the developed reference imagepasses it.

The optical scanner of the preferred embodiment uses an infraredlight-emitting diode in conjunction with a phototransistor for detectinglight from the LED which is reflected back from an adjacent surface, inthis case, the photoreceptor belt carrying the toner of the developedreference image. A standard table is provided in read only memory whichcorrelates the voltage at the output of the optical detector with theproper amount of toner of the particular process color of the referencedimage which should be present to properly develop the reference image.Again, if inadequate toner is detected, either the corona voltage may beadjusted or it may be an indication that the supply of toner is low ornot operating properly.

The preferred form of the toner deposition detector in the presentinvention modulates the LED at a particular predetermined frequency andfilters and rectifies the output of the optical detector in order tooffset the effects of ambient temperature on the infrared photodetectorto assure that the optical detector is accurately reading only lightreflected from the light-emitting diode.

A similar arrangement for detecting toner deposition on the intermediatetransfer belt is used to test for the percent of toner transferred fromthe photoreceptor to the intermediate transfer belt. If the percentageof transfer changes, alterations to the surface charge of the transferbelt may be made to improve the transfer characteristics.

Additionally, the present invention includes a plurality of gravity fedtoner modules of novel design. The toner module of the present inventionregulates the thickness of toner/carrier which passes out of the tonermodule over the photoreceptor belt so that toner does not fall onto thephotoreceptor belt under the influence of gravity.

Additionally, the present invention provides a color print engine whichis specifically designed to be interfaced to a plurality of image sourcesubassemblies including a photocopier source subassembly and a laserprinter source subassembly. In order to achieve this, the presentinvention provides a conventional serial RS232 to interface connectionbetween the print engine and the particular subassembly selected. Incopiers embodying the present invention, the serial interface isconnected to a keyboard for providing user selectable inputs.Additionally, an interface is provided from the print engine of thepresent invention to provide digital information to a copier subassemblyabout the position of the photoreceptor belt and signals which areappropriate for controlling the optical sweep of an original. The samedigital interface is also connected to a laser printer image sourcesubassembly for appropriately synchronizing the raster scan of the laserand movement of the photoreceptor belt. Firmware in read only memoriesof the print engine controller is also charged when a laser lightsource, rather than a copier optics bench is used.

The toner modules of the present invention are specifically designed tobe downwardly facing and thereby gravity fed. The desire to have arelatively large number of toner modules (five in the preferredembodiment) led to the need to design a practical downwardly facingtoner module which could be used in a full color electrophotographicprint engine.

In one embodiment of the improved toner modules of the presentinvention, a conventional decorator roller having a magnetic core of atype known to those skilled in the art is used. A supply of tonermaterials is placed in a hopper above the mixing chamber. When a sensorwithin the mixing chamber detects a need for additional toner, a gate isopened and a predetermined amount of toner materials falls under theinfluence of gravity into the mixing chamber. This eliminates the needfor lifting augers commonly found in prior art upwardly facing tonerdeposition assemblies.

Above the decorator roller is a second and novel mixing roller which isconstructed similarly to the decorator roller, in that it has an outersleeve which rotates around a magnetic core. However, the orientation ofthe magnetic poles within the core of the mixing roller are arrangedsuch that toner and carrier materials tend to fall off the downwardtraveling side of the roller and are lifted up off of the upwardtraveling side of the decorator roller onto the upward traveling side ofthe mixing roller.

According to a second embodiment of the novel toner modules of thepresent invention, the magnetic core of the decorator roller is arrangedso that it serves a conventional function when in its normal position.Apparatus for rotating the core of the decorator roller through apredetermined angle is provided. When the core is rotated, thecombination of the pole configuration in this core and the poleconfiguration in the core of the mixing roller causes the carrierparticles, and thus the toner attached to them, to continue to rotatearound the mixing roller and not fall onto the surface of the decoratorroller. Additionally, when the core of the decorator roller is rotatedto the closed position, its magnetic poles are oriented so that acenterline between two adjacent poles is pointed downwardly toward thephotoreceptor belt. This causes the flux lines of the magnetic field tobe substantially parallel to photoreceptor belt and greatly reduces theprobability that any carrier particles and toner on the surface of thebelt, resulting from a condition of carrier pull from another module,will be lifted onto the surface of the decorator roller, thuscontaminating the toner within the module.

Therefore, generally stated, it is the object of the present inventionto overcome the drawbacks in prior art color print engines recitedabove.

More specifically, it is an object of the present invention to providean improved color print engine which may be produced much lessexpensively than prior art machines, primarily due to the elimination ofcritically machine mechanical components which are replaced bynon-critical mechanical components that are driven by independentmotors, and for which the entire system is synchronized and operated bya precision digital electronic controller.

It is a further object of the present invention to provide a color printengine using flexible belts for carrying a photoreceptor and creatingcomposite images, which belts have a heretofore unacceptable mechanicaltolerance of their respective lengths, which is compensated by aprecision digital electronic controller.

It is a further object of the present invention to provide a color printengine which uses a dedicated monochrome toner in place of the standardprocess toners for making monochrome copies.

It is a further object of the present invention to provide anelectrophotographic print engine in which the rate of image travel onthe photoreceptor belt is independent of and different from the rate oftravel of an image receptor through the fuser mechanism to createincreased dwell time without sacrificing copy output rate.

It is a further object of the present invention to provide a colorelectrophotographic print engine of simple architecture which uses aplurality of the laterally spaced-apart downwardly opening, gravity fedtoner modules.

It is a further object of the present invention to provide a colorelectrophotographic print engine using a flexible belt for carrying thephotoreceptor and a flexible intermediate transfer belt for developingcomposite images which uses electrostatic forces between the two beltsto assist in maintaining proper registration during the transfer ofcolor component images to the composite image on the intermediatetransfer belt.

It is still a further object of the present invention to provide animproved color copier which includes energy management features allowingthe copier to be operated from a conventional 120 volt 15 amp branchcircuit.

It is still a further object of the present invention to provide a colorprint engine which is usable with interchangeable image sourcesincluding a photocopier image source and a laser raster scan imagesource.

That the present invention satisfies these objects, and overcomes thedrawbacks of the prior art, will be appreciated from the detaileddescription of the preferred embodiment below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the preferred embodiment of the presentinvention.

FIG. 2 is a perspective view of the outside of the preferred embodimentof the present invention atop a roller cart.

FIG. 3 is a perspective view of the preferred embodiment with sidepanels down, and the optics bench and image development sections lifted.

FIG. 4 is a perspective view of the paper handling assembly of a firstpreferred embodiment of the present invention.

FIG. 4A is a perspective view of the paper handling assembly of a secondpreferred embodiment of the present invention.

FIG. 5 is an elevational diagram of major components of the imagedevelopment system of a first preferred embodiment.

FIG. 6 is an elevational diagram of the major components of the imagedevelopment system of an alternate preferred embodiment.

FIG. 7 is a plan view of the lower side of the platen of the preferredembodiment showing color reference bars.

FIG. 8 is a partially schematic, partially block diagram of the tonerdeposition sensor of the preferred embodiment.

FIG. 9 is a perspective view of a typical photoreceptor belt used in thepreferred embodiment.

FIG. 10 is an elevational view of a representative light chopper used inthe servo system of the preferred embodiment.

FIG. 11 is a block diagram of the overall electrical control apparatusfor the preferred embodiment.

FIG. 12, consisting of FIGS. 12A and 12B, is a schematic diagram of thedigital controller of the preferred embodiment.

FIG. 13 is a block diagram of the digital servo system used to controlvarious motors in the preferred embodiment.

FIG. 14 is a partially cut away side elevation of the improved fuser ofthe preferred embodiment.

FIG. 14A is a detail drawing showing the contact areas between variousrollers in the fuser of FIG. 14.

FIG. 15 is a section view of a first preferred embodiment of the tonerdeposition modules of the present invention.

FIG. 16 is a section view of an alternate preferred embodiment of thetoner deposition modules of the present invention.

FIG. 17 is a diagram representing in cross-section, the magnetic poleorientation for the cores of the decorator roller and the mixing rollerof the embodiment of FIG. 15.

FIG. 18A is a diagram representing, in cross-section, the magnetic poleorientation of the cores of the decorator roller and mixing roller ofthe embodiment of FIG. 16 when in the open position.

FIG. 18B is a diagram representing, in cross-section, the magnetic poleorientation of the cores of the decorator roller and mixing roller ofthe embodiment of FIG. 16 when in the closed position.

DETAILED DESCRIPTION

Turning now to the various drawing figures, in which like numeralsreference like parts, a detailed description of the preferred embodimentwill be provided. A block diagram of the basic architecture of thepresent invention is shown in FIG. 1. The entire machine is operatedunder the control of a digital controller 20 which will be described ingreater detail hereinbelow. Controller 20 includes a master timingsource derived from a 12 megahertz crystal clock 21. Controller 20includes a microprocessor having instructions located in read onlymemory 22. Read only memory 22 is shown as connected by a plug connector25 to indicate that ROMs embodying ROM 22 can be selectively replaceddepending on the particular configuration of an embodiment of thepresent invention. Also random access memory 26 is connected tocontroller 20 for storing various values and states, described ingreater detail hereinbelow.

The source of images for the present invention is shown by block 28marked optical or laser bench. This is to indicate that the light signalsource for input to the present invention is selectively replaceable andcan include an optical bench in the case of the preferred embodimentperforming as a copying machine, or a laser bench in the case ofembodiments instructed as laser printers. In the preferred copierembodiment, a motor 29 is included to drive an optical scanner. Two-waydata communication takes place between controller 20 and bench 28 vialines 30 and 31.

Two main subsystems of the preferred embodiment are the photoreceptorbelt subsystem 32 and transfer belt subsystem 35. Each of thesesubsystems includes conceptually similar components. Each subsystem isdriven by a motor, 36 and 37, respectively. Motor 36 drivesphotoreceptor belt 38 and motor 37 drives transfer belt 39. Motiontransducer 40 responds to movement of motor 36 as indicated by dashedline 42. Similarly, motion transducer 41 responds to motor 37 asindicated by dashed line 43.

In the preferred embodiment, these devices include light choppers whichwill be described in greater detail hereinbelow. Motion transducers 40and 41 provide output on lines 44 and 45, respectively, to controller20. Controller 20 provides pulse width modulated input signals tocontrol the speeds of motors 36 and 37 on lines 46 and 47, respectively.

Reference detectors 48 and 49 determine the passage of predeterminedreference points past a sensor and are used together with precise timingsignals derived from crystal clock 21 to control movement of belts 38and 39 to achieve proper registration, as discussed above in connectionwith the summary of the invention. Outputs from reference detectors 48and 49 are provided to controller 20 on lines 50 and 51, respectively.

The paper drive and paper path in the present invention is alsocontrolled from digital controller 20. Line 52 represents a plurality ofcontrol signals to a plurality of clutches 55 which are mechanicallylinked to a motor 56 as shown by line 57. Clutches 55 activateconventional paper picking and movement functions represented by block59 and line 60.

As described hereinabove, there is a space represented by dashed line61, within the paper path between the output of the point at which acomplete image is transferred to a sheet of paper, and the input offuser 62. This spacing is suggested by dashed line 61.

Fuser 62 is driven by a motor 65 as indicated by dashed line 66. Motor65 is also controlled by digital controller 20 through signals providedon line 67. Control signals to the fuser are provided from controller 20on line 68 and information on the status of the fuser, primarilytemperature and operating speed, are returned to digital controller 20on line 69.

As will be apparent to those skilled in the art, FIG. 1 represents thecontrol architecture of the print engine of the present invention. Animportant aspect of the architecture of the print engine of the presentinvention may be readily appreciated from viewing FIG. 1. Subsystems 32and 35 are two substantially identical subsystems for driving the beltsused in the present invention to create images. Each subsystem providesprecise digital timing signals about the motion of its associated beltto digital controller 20. Also, motors 36 and 37 driving each respectivesubsystem are driven at speeds controlled by digital controller 20,preferably through the use of pulse width modulated signals. Informationabout the present position of predetermined points on the belts areprovided to controller 20 by reference detectors 48 and 49.

These signals are used, in connection with precise timing signalsderived from crystal clock 21, to determine the lengths of belts 38 and39 as well as their present position at any given point in time.Therefore, as pointed out in the summary of the invention, belts 38 and39 can be, and in practical embodiments of the present invention are, ofa nominal length plus or minus a given intolerance for which digitalcontroller 20 can compensate. An important aspect of this giventolerance is that it is much greater than allowable tolerances in priorart mechanically linked color electrophotographic print engines.

In the preferred embodiment, controller 20 uses the motion ofphotoreceptor belt 38 as a master reference and adjusts operation of theother subsystems of the preferred embodiment to be properly synchronizedwith the motion of the photoreceptor belt.

Another aspect of the preferred embodiment represented by FIG. 1 is thefact that the non-critical mechanical motion functions such as pickingand feeding of paper are under the control of a common motor 56 and aplurality of clutches 55, all of which are mechanically linked in amanner similar to how such components operate in prior art machines. Thesignificant aspect of this arrangement is that the non-criticalmechanical movement elements are operated by a single motor and aplurality of clutches, whereas the critical elements for registration;movement of photoreceptor belt 38 and transfer belt, 39 are made underprecise control of digital controller 20.

Furthermore, fuser 62 is driven by an independent motor 65. As notedhereinabove, this allows a sheet of image receptor, preferably paper orplastic transparency material, to be driven through the fuser at aslower rate than images are transferred between belts 38 and 39 duringcolor electrophotography. This is because the fuser must operate on onlyone composite image, for every three images created on photoreceptorbelt and transferred to transfer belt 39. This independent operationallows the dwell time in the fuser to be increased without slowing downthe copying rate of the machine or unduly increasing the fusertemperature. This arrangement also allows fuser motor 65 to be driven atsubstantially the machine speed of belts 38 and 39 when the copier isoperating in a monochrome mode.

The replaceability of bench 28 is further suggested by the blockrepresentation of plug connector 33. In this connection it should beunderstood that while the significance of signals on lines 30 and 31will vary depending on whether a laser or optical bench is mounted overthe print engine of the present invention, controller 20 is designed sothat an adequate number of digital interface lines are provided toembody lines 30 and 31 so that either type of light source may be used.

Turning next to FIG. 2, a perspective view of a copying machineembodying the preferred embodiment of the print engine of the presentinvention is shown. In FIG. 2, the copying machine embodying the presentinvention sits atop a conventional rolling cart 75. It should beapparent from FIG. 2, the copying machine of the preferred embodiment isof substantially the same size as a conventional desk top monochromecopier, as was discussed in the background of the invention section.

The outside of the copier includes hinged side panels 76 and 77 and aconventional output tray 78. A cover 79 is hinged at point 80 so that itmay be lifted off a platen (not shown) for placement of an original tobe copied on top of the machine. A plurality of ventilation slots 81 arealso provided in the end of a copier. A keyboard and display area 85appears on the upper surface of the machine. This area includes aplurality of keys 86 and a display panel 87.

The user controls operation of the machine via inputs entered throughswitches 86. Display 87 provides the user with conventional informationabout the mode of operation selected, paper size, number of copiesselected, number of copies made, and the like. Among the modes ofoperation which may be selected via keyboard 85 are whether the machinewill operate in a monochrome or full color modes, which are described ingreater detail hereinbelow.

FIG. 3 shows a perspective view of the preferred embodiment in an openposition. Side panel 76 is lowered, and an upper portion of the machineis raised. The upper portion of the machine is hinged about an axisshown as 110 and is supported by a pair of spring-loaded telescopingrods 111a and 111b. The lower part of the machine is the paper handlingassembly, generally indicated at 112. The upper portion of the machineincludes an image developing portion 115 and optics bench 28 which isjoined to the image developing portion at 116.

As described in connection with FIG. 1, the print engine of thepreferred embodiment is specifically designed to be usable with variousoptical sources such as optical bench 28. It should be noted thatoptical bench 28 is bolted to image developer section 115 and isinterconnected electrically through a plurality of plug connected wires.Therefore, it should be apparent that the preferred embodiment of thepresent invention has been specifically designed to provide a printengine which is usable with replaceable image sources.

The fuser mechanism 62 is located in front of output tray 78.

A solenoid 117 is used for periodically applying silicone oil to rollerswithin fuser assembly 62, as described in connection with FIG. 14.

Also visible in FIG. 3 are photoreceptor belt 38 and transfer belt 39.Photoreceptor belt 38 is driven around a roller at 118 by a motor (notshown in FIG. 3). Transfer belt 39 is driven by motor 37. A conventionalexpose station over photoreceptor belt 38 is shown at 120. This includesan exposure corona 121 of a type well known to those skilled in the art.The corona is used for providing an electrostatic charge tophotoreceptor belt 38. It should be understood that in the preferredembodiment, photoreceptor belt 38 rotates in a direction indicated byarrow 122 and thus transfer belt 39 rotates in a direction indicated byarrow 125.

A discharge corona 126 is also provided at a cleaning station alongphotoreceptor belt 38. Any toner which remains on photoreceptor belt 38after a developed image has been transferred to transfer belt 39 iscollected in a receptacle 127 in a conventional manner.

An open space 135 is provided above photoreceptor belt 138 for housingtoner modules. In FIG. 3, three toner modules 136a-136c are showninstalled within area 135. Area 135 is proportioned to accept up to atotal of five toner modules. As noted above in the summary of theinvention, it is preferable to have toner modules 136a-136c hold tonermaterials for the three color process toners and to use a fourth tonermodule (not shown in FIG. 3) for monochrome copying. The space for thefifth toner module is provided so that custom colors, not reproduceablethrough the use of available process colors, may be used either inmonochrome copying with a copying machine of the type shown in FIG. 3 orto color predetermined portions of an image in a laser printer embodyingthe print engine of the present invention.

In the preferred embodiment, photoreceptor belt 38 is thirty-eightinches long and transfer belt 39 is nineteen inches long. The importanceof this relationship is that the length of transfer belt 39 needs tonominally be an integer submultiple of photoreceptor belt 38. As used inthis specification, the concept of an integer submultiple includes asubmultiple of one, wherein belts 38 and 39 are of equal length.

It should be noted from inspection of FIG. 3 that it is the use offlexible belts in a full color print engine of the preferred embodimentwhich is primarily what allows the machine to be constructed so that itssize approximates that of a conventional convenience table top copyingmachine. As noted hereinabove in the background of the invention,construction of a reliable full color copier of this type has beenunknown in the prior art.

FIG. 4 is a detail drawing showing the paper handling subassembly 112for the embodiment of FIG. 3. For the most part, this subassembly isconventional. The paper handling subassembly includes two paper traysshown at 145 and 146. Each of the paper trays has associated therewith,a paper picker 147 and 148, respectively. The paper picker assembliesare driven by gears 149 and 150, respectively. Also, a pair of papergates 151 and 152 are provided for each paper tray and are driven fromgears 155 and 156, respectively.

All the above-referenced gears, are driven by a common drive chain 157and are engaged by operation of electromechanical clutches (not shown)in a conventional fashion. Within paper tray 146, a plurality of sheetsof paper 158 are shown.

In the preferred embodiment of the present invention, paper is the mostcommonly used image receptor for the final image produced by a machine,but other image receptors may be included. In particular, the inventorsof the present invention believe that it is important that the printengine of the present invention be usable with plastic transparencymaterial since color plastic transparencies are particularly useful inconnection with overhead projectors for making effective presentationsof materials to an audience.

In the embodiment shown in FIG. 4, fuser assembly 62 is driven by asecond chain 160 mechanically linked through gears 161 and 162 on acommon shaft 165. All of this is in turn driven by a motor shown inphantom as 166.

As noted hereinabove, the most preferred form of the present inventionis one for which a separate motor is used for driving paper pickers 145and 146 and paper gate 151 and 152. This embodiment is shown in FIG. 4A.In FIG. 4A, the paper picking and gating assembly is identical to thatshown in FIG. 4. All of this is driven by a common drive chain similarto drive chain 157 shown in FIG. 4, which is referenced as 157' in FIG.4A. In FIG. 4A, the paper picking and gating assembly is driven by amotor 56. An improved fuser assembly 62 as shown in FIG. 4A is laterallyspaced apart from paper gate 152. The structure of the improved fuserassembly is described in detail hereinbelow. A plurality of rubbercarrying belts 153 carry a sheet of paper bearing an image transferredfrom the transfer belt toward fuser mechanism 62. The dimension line dis shown between a point 154 on one of belts 153 and the entrance tofuser mechanism 62. Point 154 represents the point at which a sheet ofpaper bearing a developed image exits its contact with the transferbelt. Thus, when the trailing edge of a sheet of paper leaves thetransfer station, it is laid on belts 153 and is not in contact witheither transfer belt 39 or the driving mechanism of fuser 62. Therefore,it is important that distance d be selected so that they hold an entiresheet of paper of at least one commonly used sheet size.

As pointed out above, paper handling assembly 112 is driven by motor 56and fuser assembly 62 is independently driven by motor 65. In the mostpreferred form of the present invention, illustrated in FIG. 4A, motor65 drives fuser mechanism 62 at a rate which produces a slower linearrate of travel of the sheet of paper through the fuser and the linearmachine rate at which photoreceptor belt 38 of transfer belt 39 are run.This is possible because the fuser is spaced apart from the output endof the transfer station by at least predetermined distance d.

When the preferred embodiment is in a color copying mode, three separaterevolutions of transfer belt 39 are required to create a completedeveloped composite image. When multiple copies are being made, thismeans that three additional revolutions of the transfer belt arerequired to make the next subsequent copy. Therefore, in prior art colorcopying machines in which the fuser operates at the machine rate, thefuser is idle approximately two-thirds of the time. By adopting thearrangement shown in FIG. 4A, the present invention allows a sheet ofimage receptor bearing the composite developed image to be driventhrough the fuser more slowly than the machine rate at which copycreation is taking place and thereby increases the dwell time of thecopy in the fuser without slowing down the overall copy output rate.Inventors of the present invention also believe this has an addedbenefit of providing the user with an illusion that the machine isalmost continuously generating copies rather than the normal relativelylong period between output of copies experienced in conventional colorcopying machine.

Before proceeding with a description of the image developmentsubassembly of the present invention, one important aspect of thepresent invention is emphasized by viewing FIGS. 3 and 4A in conjunctionwith each other. As may be seen from inspection of these figures, belts38 and 39 in the image development subassembly are driven independentlyof the drive mechanisms within paper handling assembly 112. In thepreferred embodiment, a precision digital controller, described ingreater detail hereinbelow, is used to appropriately synchronize, and insome cases intentionally unsynchronize, operation of components of thesetwo subassemblies in order to achieve optimum results from the printengine.

Turning next to FIG. 5, a detail drawing of a first preferred embodimentof a portion of the image development subassembly is shown. FIG. 5 showsa detailed drawing of the embodiment of FIG. 3. Each of belts 38 and 39are mounted about a pair of rollers on pivot points 118 and 168 for belt38 and 170 and 171 for belt 39. Associated with the mechanical elementscarrying these belts are tension adjusting cams 175 and 176. These camsare loosened when belts 38 and 39 are to be removed, and are used toproperly tension the belts after installation.

A plurality of coronas 121, 126 and 128 are located at expose station120. Coronas 126 and 128 are sensitizing coronas used to charge belt 38prior to creation of an electrostatic image. Corona 121 is a dischargingscoratron. On the lower side of photoreceptor belt 38, an AC corona 129is located. This corona is excited with an alternating current voltagewhich tends to disperse charge remaining on the belt and thus loosen anyresidual toner particles which may be left on the belt after transfer ofthe developed image to transfer belt 39. The particles can therefore becleaned by a scraper blade (not shown) over cleaning station 127 (FIG.3).

Also shown in phantom at expose station 120 is an electrostaticvoltmeter 123 used in the copy quality monitor of the preferredembodiment.

As photoreceptor belt 38 moves past expose station 120, light fromoptics bench 28 (FIG. 3) is focused onto the belt in synchronism withits movement to provide an electrostatic image on the photoreceptorbelt. Assume for the moment that color copying is taking place. Tonermodules 136a-136c contain the three color process toners. When the firstimage is developed, an appropriate separation filter (not shown) isinterposed between a light source and the optics bench and exposestation 120 in a known manner. As belt 38 moves past expose station 120in the direction shown by arrow 122, an electrostatic image of thisparticular color component is created on belt 38 in a manner which willbe familiar to those skilled in the art, and which was described in thebackground of the invention.

Assume that the first color developed corresponds to the color containedin toner modules 136a. As a developed image moves past toner module136a, the module is activated to deposit toner materials on the chargedportions of the surface of belt 38 to provide a developed image of thiscolor component of the original. As belt 38 continues to rotate, theleading edge of the developed image eventually reaches point 178 atwhich belt 38 first makes contact with belt 39. The developed image istransferred from belt 38 to belt 39 as the belts continue to rotate andthe image passes from point 179 to point 179.

It should be noted in FIG. 5, belt 38 is urged firmly against belt 39and an open space 180 is provided behind portion of belt 39 whichcontacts belt 38. This provides a contact area between points 178 and179 which is referred to herein as the wrap of the belts. In thepreferred embodiment, the length, along the direction of travel of belt38, of the wrap area between points 178 and 179 is approximately 0.6inch. It is believed preferable to have this area be at least 1/2 inchin length.

Two mechanisms resulting from the wrap area of the belts aid in transferof the image and maintenance of proper registration. First, aconsiderable mechanical force is provided in the wrap area simply fromthe force of having belt 38 urged strongly into belt 39, as shown.Secondly, belt 39 is charged to a high positive potential by corona (notshown) so that it has approximately one kilovolt of electrostaticpotential as a result of positive surface charge. This surface chargecauses the toner material on belt 38 to be transferred to the belt, thustransferring the developed image, and furthermore creates extremelystrong electrostatic forces between belts 38 and 39.

Considering a one kilovolt surface charge potential on belt 39, and theintimate contact between the belts in the wrap area between points 178and 179, it will be quickly appreciated that a very intense electricfield strength exists in the wrap area. This helps hold the beltstightly together and prevents any slip from occurring as compositeimages are laid one over the other. Indeed, a prototype of the presentinvention has been operated with the above-recited charge conditions onthe belts with the motor driving transfer belt 39 turned off, and theelectrostatic forces between the two belts were sufficient to causecontinued rotation of belt 39 and to overcome the mechanical frictionand inertia of the components attached thereto.

Once approximately half of belt 38 has passed expose station 120 afterdevelopment of a first composite image, the second separation filter isinserted in the optical path in optics bench 28, and an electrostaticimage of a second color component of the original is exposed onphotoreceptor belt 38. Toner module 136b is activated as thiselectrostatic image passes under it in order to provide a second colorcomponent developed image. This image is likewise transferred to belt 39as a third color component image is being developed.

It should be noted that belt 38 is of sufficient length so that, forconventional 81/2×11 copies, the second electrostatic image is beingdeveloped on one portion of the belt as the first developed image isbeing transferred to belt 39. Therefore, one and one-half revolutions ofbelt 38 are required to create, develop and transfer a three-colorcomposite developed image during three rotations of transfer belt 39.

It should be noted in connection with FIG. 5, that one unique aspect ofthe architecture of the preferred embodiment is that all of tonermodules 136 are downward facing and gravity fed. The conventional wisdomof those skilled in the art of color copying indicated that downwardfeeding gravity driven toner modules would not be usable in a colorcopying machine because of the fear that excess toner would fall outonto the belt and contaminate the images. The improved toner moduleswhich allow for this architecture are described in greater detailhereinbelow in connection with FIGS. 15-18.

Once a complete composite image has been transferred to photoreceptorbelt 39, an image receptor, normally a piece of paper, is provided bypaper handling subassembly 112 (FIG. 4) to a transfer station at 185. Inthe first preferred embodiment, a flexible web shown as 186 is providedunder the roller rotating about pivot point 171 so that the paper isurged against transfer belt 39. Once transfer is complete, the paperthen moves through a lateral open space shown by dimension line d inFIG. 5.

Dimension d corresponds to the same dimension shown in FIG. 4A, and ischosen such that at least one standard size sheet of paper (81/2×11 inthe preferred embodiment) can occupy space d between the point at whichthe trailing edge of the sheet of paper last contacts belt 39, and thepoint at which the paper enters fuser mechanism 62. Therefore, once acomposite image has been transferred from transfer belt 39 to the sheetof paper, it is free from any need to have its movements synchronizedwith transfer belt 39 and can enter the fuser which, in the mostpreferred embodiment of the present invention, can be operated at aslower speed than the machine speed at which belts 38 and 39 areoperating. It should be understood that the concept of machine speed islinear rate of travel of the image carrying and developing elements.Therefore, by spacing the exit from the transfer station at 185 by atleast a predetermined distance d from the entrance to fuser 62 shown at187, the fuser may be, and is, operated at a speed independent of thespeed at which transfer belt 39 is moving.

Next, operation of the image developer when the preferred embodiment isin a monochrome mode of operation is discussed. Toner module 136dcarries a conventional black monochrome toner material. Naturally, whenthe copier of the preferred embodiment is in a monochrome mode ofoperation, no color separation filters are inserted in the optical path.

As an electrostatic monochrome image was created on belt 38, the imagemoves under toner module 136d which applies monochrome toner toelectrostatic image to provide a developed monochrome image. When thedeveloped image reaches the wrap section between the belts at point 178,it is transferred to the transfer belt 39 in the manner describedhereinabove.

As the leading edge of the transferred developed image reaches paperentrance station 185, a piece of paper is provided by the paper handlingmechanism and thus the developed monochrome image is transferredimmediately to the paper. When a position on photoreceptor belt which isapproximately half way from the point at which the first monochromeimage was created reaches the expose station, a second image of theoriginal (assuming the user has selected multiple copies) is exposed onthe photoreceptor belt. This image also passes under toner module 136dwhere it is developed.

Considering the nominal two to one ratio between the lengths of belts 38and 39, it will become apparent that transfer belt 39 will have madeapproximately one complete revolution at the time the leading edge ofthe second developed image arrives at point 178. The above describedprocess repeats itself and a sheet of paper is provided to transferstation 185 when the leading edge of the transfer developed image onbelt 39 arrives there.

What should be apparent from the foregoing discussion is the fact thatwhen the preferred embodiment is in a monochrome mode of operation,developed images from belt 38 are transferred to belt 39 and thenimmediately to an image receptor, normally in the form of a sheet ofpaper or a plastic transparency. The timing of exposure and developmentof monochrome images on belt 38 is such that it substantially duplicatesthe timing for exposure of color component images when the machine is ina color mode of operation. Therefore, timing of the transfer of themonochrome images to belt 39 also duplicates that of the color process.The main difference, insofar as operation of belts 38 and 39 isconcerned, is that each developed image is transferred immediately frombelt 39 to a sheet of paper as belt 39 makes one complete revolution.Therefore, it will be apparent that the copy output rate of thepreferred embodiment in the monochrome mode of operation isapproximately three times the copy output rate of the machine in thecolor mode.

In the preferred embodiment, the nominal machine speed of belts 38 and39 is ten inches per second. Therefore, the preferred embodiment willproduce slightly more than six full color copies per minute in the colormode. In the monochrome mode, the preferred embodiment can produce overeighteen copies per minutes of 81/2×11 originals, and thus it producesone copy in slightly less than four seconds. From this, it can beappreciated that the preferred embodiment indeed achieves the object ofthe invention recited above of providing a full color copier which alsohas a monochrome mode of operation with a copy output rate that isacceptable to make it the standard office copying machine for many smalland medium size office environments.

The use of a separate toner module 136d for a dedicated monochrome toneris used to overcome two drawbacks of prior art copiers citedhereinabove. The first drawback was that in copying a monochromeoriginal, prior art machines went through the unnecessary steps ofshooting three substantially identical composite images of themonochrome original through the separation filters. This arrangement, ofnecessity, causes the prior art color copiers to produce monochromecopies at the same rate as they produce color copies.

Secondly, the use of process colors for producing black monochromecopies has long been recognized as providing copies of inferior qualityinsofar as the black color content is concerned. Additionally, evenslight problems in registration using process black cause a decrease insharpness of the image since it is smeared due to slight offset of thecomposite images and furthermore, the outline fringes of such compositeimages will show traces of one of the process colors.

Naturally, when the preferred embodiment is in its monochrome mode ofoperation, the extra time available for increasing dwell time in thefuser, described hereinabove, is lost. Therefore, the fuser mechanism 62must operate at a linear rate of approximately ten inches per secondwhich is the machine rate at which belts 38 and 39 are driven. However,two factors offset this effect. First, since a dedicated monochrometoner is used, it is possible to select monochrome toner materialsrequiring less dwell time in a fuser of a given temperature in order toproperly fix the toner materials. Additionally, since the monochromeimages are exposed without the separation filters in the optical path,the light source used in exposing monochrome copies is selected to be ofone-half the intensity of the light source used when exposing colorcomponent images. Therefore, when in a monochrome mode of operation, theoptics bench draws much less power than it does when the machine is in acolor mode of operation and more power can be dedicated to increasingthe fuser temperature as the dwell time decreases.

In the preferred embodiment, two 400 watt lamps are used in the opticsbench and only one of these is turned on when the machine is in amonochrome mode of operation.

As noted above, a fifth toner modulator 136e is provided for creatingcopies with custom colors. As used in this specification, custom colorsinclude colors which may not be accurately reproduced within thelimitations of the three color process colors contained in toner modules136a-136c. Many companies and organizations have adopted specific colorsused in connection with logos, trademarks and the perfer, and like forsuch colors to be displayed on material they distribute. Methods forcreating custom monochrome toners for reproducing a specific customcolor are well known to those skilled in the art.

In the preferred copier embodiment of the present machine, a customcolor toner in toner module 136e may be used to print either an entiredocument when a machine is in a monochrome mode of operation or may beused to customize stationary or the like which may then be reinsertedinto the paper trays of the machine to have other material reproducedthereon by the processes described above.

For example, if a user desires to include a logo with a custom color ona plurality of sheets of paper upon which copies are to be made, amonochrome image of the logo may be placed on the optic bench. Themachine is placed in a monochrome mode of operation with the customcolor selected. Multiple copies may then be made which will provide thelogo bearing the custom color in the position dictated by the monochromeimage on the original. Once this is accomplished, the resultant sheetsof paper bearing the custom colored logo may then be reinserted into thepaper tray and either monochrome or full color material may be copiedonto these sheets.

In laser printer embodiments of the present invention, a four coloroperation is used to provide custom coloring in particular areas of aprinted image. In the case of the laser printer, it is up to the creatorof the software driving the laser printer to define which areas of theprinted page will bear particular colors. The software need only be ableto specify a printed image area to be used which will bear the customcolor and provide appropriate control signals to the embodiment of thepresent invention.

When this process is run, a four color copying process takes place inwhich three process color composite images are used in their normalfashion to create full color material on the printed page. Subsequently,a single monochrome composite image is exposed by the laser beam onappropriate areas of photoconductor belt 38, and this fourth compositeimage is developed under tone module 136e and transferred as part of anoverall four layer composite image to transfer belt 39. Once the fourseparate images have been transferred to belt 39, the overall compositeimage is transferred to a sheet of paper which is provided to the fuser.

Additionally, an extremely useful mode of operation in one in which thecustom color is used to color a particular area of the printed page (forexample a company logo) and black monochrome toner is used to print thebalance of information on the page. In this mode of operation, twoseparate composite images are created in response to signals from thesoftware providing input to the laser printer. One composite imagedefines the area to be colored by the custom color which is developed bythe toner in module 136e and transferred to belt 39. Subsequently, thelaser bench develops an electrostatic image corresponding to the blackmonochrome information to be included on the printed page. This isdeveloped under toner module 136d and transferred as a second compositeimage onto transfer belt 39. When this transfer has been completed, thenext time leading edge of the two component composite image on belt 39arrives at transfer station 185, a sheet of paper is provided at thislocation and the two color composite image is transferred.

It will be readily appreciated that in this mode of operation, twoseparate monochrome images are developed which are overlaid onto belt 39as a composite image. Naturally, it is assumed that the user will takecare to make sure that the black monochrome images and the custom colormonochrome images are spatially separated. Since two images must becreated for each copy when in this mode of operation, the preferredembodiments can produce slightly more than nine output copies perminute.

Turning next to FIG. 6, an alternate preferred embodiment of thestructure driving transfer belt 39 is shown. The belt is driven around aroller of circular cross section rotating about an axis shown as 170' toindicate its correspondence to axis 170 in FIG. 5. However, at the lowerportion of the assembly, two additional rollers 190 and 191 rotate aboutaxes 195 and 196. In this embodiment, paper introduced to a transferstation, indicated as 185', strikes a deflector plate 197 and is broughtinto contact with transfer belt 39 under roller 191. A corona 198charges the paper to transfer the toner particles constituting thedeveloped image on belt 39 to the paper. The paper then exits thetransfer station at 199.

In the embodiment of FIG. 6, the space between entrance point 185' andexit point 195 of the transfer station has a length indicated bydimension line 210. In the preferred embodiment shown in FIG. 6, length210 is approximately 21/2 inches. The relatively large area of contactbetween transfer belt 39 and the paper provided by the three rollerembodiment of the transfer station shown in FIG. 6, takes advantage ofthe electrostatic forces between belt 39 and the paper to keep the paperfrom slipping, and thus smearing an image, and to assure a highpercentage of transfer of the toner to the resultant copy.

Certain aspects of the novel copy quality monitoring system of thepresent invention will now be described in connection with FIGS. 7 and8. FIG. 7 shows the glass platen 220 for accepting originals in copymachines embodiments of the present invention. The view of FIG. 7 showsplaten 220 as viewed from the interior of the machine. As will beapparent to those familiar with copying machines, platen 220 lies undercover 79 (FIG. 2).

On the right-hand end of platen 220 are four reference color bars221-224. These bars are, respectively, white, magenta, cyan, and yellow.In FIG. 7, bar 222 has been lined for red and bar 223 has been lined forblue to indicate magenta and cyan, and bar 224 is lined for yellow.These four color bars constitute a reference means for providing aplurality of electrostatic test images on the photoreceptor belt 39.When the copying machine is to be calibrated to test for copy quality,the optical system of optics bench 28 is configured to focus images ofreference bars 221-224 onto the photoreceptor belt as an image is shotthrough each of the successive separation filters in the optical system.The digital controller of the preferred embodiment, described in greaterdetail hereinbelow, keeps track of where each of the images of referencebars 221-224 is located as each image is shot.

First, a non-contact electrostatic volt meter is positioned overphotoreceptor belt 38 at 123 (FIGS. 5, 6). When digital controller 20(FIG. 1) detects that the position on photoreceptor belt 38 bearing theimage of one of the color bars is passing under the electrostatic voltmeter, a reading of the voltage is made. Note that, for each successivefilter used to create such an image, a reference bar on an electrostaticimage should exist from the contribution of white bar 221 as well as theparticular one of reference bars 222 through 224 which corresponds tothe filter through which the image is shot.

It is believed by the inventors of the present invention that it isknown in the art to test the electrostatic voltage on a photoreceptorbelt in a copying machine under conditions of no light and intenseexposure to white light. However, it is believed that the testing ofelectrostatic voltages present on electrostatic images created byshooting a reference color standard through the separation filters in acolor copying machine is new in the art.

If the measured electrostatic voltage is not within an acceptable range,digital controller 20 goes to a look-up table contained in memory andmakes an appropriate adjustment to the input voltages charging thephotoreceptor belt coronas (FIGS. 5, 6). The images are the reshot untildigital controller 20 detects an acceptable output voltage level fromthe electrostatic volt meter.

In addition to electrostatic measurement of electrostatic images createdin response to reference color bars 221-224, the preferred embodimentadditionally develops these images, and tests for adequate tonerdeposition for each component color. First, it should be noted that thisarrangement allows digital controller 20 to detect defects in operationof toner modules 136 even when the corona voltages used on the belts areappropriate for the current ambient conditions of humidity.

When executing this test, each electrostatic image of a reference colorbar passes under the appropriate one of toner modules 136 (FIG. 5, FIG.6) and is developed in the normal fashion. As each reference image isdeveloped, it passes under an optodetector station (illustrated in FIG.8. FIG. 8 is a partially block and partially schematic diagram of theoptical detector used in sensing toner deposition in the preferredembodiment. An LED 226 is driven by a 15 kilohertz signal on line 227which is provided from digital controller 20. LED 226 is placed overphotoreceptor belt 38 and is excited by the signal on line 227 when thedeveloped reference image including toner shown as 228, passes under theLED. Light from LED 226 is indicated by dashed line 229 as strikingtoner 228, and some of this light is reflected, as indicated by dashedline 230. However, because of the particulate nature of toner 228,significant amounts of the light 229 illuminating toner 228 arescattered as indicated by arrows 231 in FIG. 8. The unscattered lightrepresented by line 230 illuminates the base of a phototransistor 232.

As will be apparent to those skilled in the art, modular circuitsincluding photodiode 226 and phototransistor 232, together withappropriate focusing optics, are commonly available and are what areused in the preferred embodiment to embody the corresponding elementsshown in FIG. 8. Thus, it will be appreciated that these devices arearranged to physically be directly over photoreceptor belt 38 to bothilluminate the belt and to detect reflected light therefrom.

Phototransistor 232 is arranged in a simple common emitter configurationtaking output at its collector, which is coupled through a capacitor 235to a high pass filter 236. Capacitor 235 prevents any DC component fromthe collector current of transistor 232 from reaching the input of highpass filter 236. Filter 236 removes any 60 and 120 cycle noise from thesignal which appears at its output on line 237. From line 237, thesignal is rectified by full-wave rectifier shown as 238. A pulsedunipolar output signal is provided on line 239 which is passed to theinput of a low pass filter 240.

By proper selection of the filter order and cut-off frequencies forfilters 236 and 240, which is well within the ordinary level of skill inthe art, a DC signal is provided on line 241 which has a magnitude thatis proportional only to the 15 kilohertz component of the input signalto the base of transistor 232. The DC voltage level on line 241 isprovided to an analog multiplexer 374 (FIG. 12) which forms a part ofdigital controller 20.

In the preferred embodiment of the optical detector of the presentinvention, LED 26 produces, and transistor 232 responds to, infraredlight. The thickness of the deposited toner 228 on belt 38 is detectedprimarily by measuring the scattering phenomena represented by lines231. Up to a certain minimum acceptable toner thickness, the more tonerpresent under LED 226, the more the infrared radiation will bescattered. Therefore, the lower the level of infrared radiationilluminating the base of transistor 232, the greater the amount of tonerdeposited. This is because photoreceptor belt 38 is a very smoothsurface of relatively high reflectivity. Thus, if the toner is toothinly deposited, or absent altogether at certain locations under LED226, a very large percentage of the emitted infrared radiation from thediode will strike the base of the transistor.

There are two significant advantages to the arrangement of FIG. 8described hereinabove. The selection of infrared wavelengths for theradiation used to detect toner deposition provides a condition in whichthe reflected radiation is substantially insensitive to the color oftoner materials 228. Thus, there is a direct correlation between the 15kilohertz component of the input signal to the base of transistor 232and the thickness of toner 228, substantially independent of the toner'scolor.

However, the use of an infrared photodetector 232 has a potential ofrendering the optical detector sensitive to variations in ambienttemperature within the machine. Therefore, the use of the 15 kilohertzsignal to excite LED 226, and the 15 kilohertz detector circuitry shownon the right-hand side of FIG. 8, removes any effects of ambienttemperature from the infrared measurement. It should be understood thatwhile use of infrared radiation is preferred, it is also possible toconstruct embodiments of the toner deposition detector of FIG. 8 usingradiation having frequencies of visible light. Thus, as used in thisspecification and the appended claims, reference to a light sourceembodied by LED 226 will be understood to include radiation within thevisible light spectrum as well as radiation in adjacent spectra,including the infrared.

In addition to use of the optodetector of FIG. 8 over belt 38, anidentical detector is also provided over belt 39. This detector is usedto test the quantity of toner transferred from belt 38 to belt 39 whenconducting print quality tests using developed images of reference bars221-224 (FIG. 7). Appropriate adjustments to the charged state of belt39 are made by the digital controller in response to the measuredamounts of toners on the developed reference images which aretransferred to belt 39.

Before describing the digital controller of the preferred embodiment indetail, it is appropriate to discuss two important mechanical componentswhich interact with the digital controller.

Turning first to FIG. 9, an illustration of belt 38 is shown. Belt 38has a seam 250 in it created during original fabrication. It should beunderstood that some belts available to embody photoreceptor belt 38have no physical seam, since they are originally fabricated as endlessbelts. However, references to a seam area on belts, as that term is usedin this specification, references a predetermined area on the belt whichis treated as having a seam by the digital controller, even if nophysical seam is present. An index notch 251 is shown on one side ofbelt 38. Extending over this same edge of belt 38 is an optodetectorpair 252. Optodetector 252 is a conventional device which includes alight-emitting diode illuminating the surface of belt 38 and aphototransistor positioned under the belt. Each time index notch 251passes under the LED of optodetector 252, an output signal from thedetector's phototransistor is provided on wires 255.

It should be understood that, while not separately shown, a similarindex notch and optodetector are used in connection with belt 39.Additionally, belt 39 also has a seam area as defined hereinabove.

As will become apparent from the description of the digital controller,index notches 251 on belts 38 and 39 are used to detect each respectivebelt reaching a predefined home position.

FIG. 10 is a representation of the light chopper wheel used on each ofdrive motors 36, 37, and 65. Each of these light choppers consists of awheel structure 256 which is mounted on motor shaft 257 of therespective motor with which the wheel is associated. Each wheel has aplurality of teeth, indicated generally at 258. An optodetector 259 ispositioned over the edge of wheel 256 so that teeth 258 sequentiallymake and break the optical path between the light source and thephotodetector within optodetector 259. Such an arrangement is commonlyused in other electromechanical devices as a speed transducer, and it isused as such in the present invention. The output from optodetector 259is an alternating electric voltage having a frequency which isproportional to the rotational speed of wheel 256.

It should be understood that motors 36, 37 and 65 in the preferredembodiments are DC motors which have reduction gears provided betweenthe motor's output shaft and the shafts upon which the rollers describedherein carrying belts 38 and 39, and the shaft of the main fuser drive,are mounted. The gear ratios are selected so that in response to anappropriate DC voltage input signal, the motors will drive thecomponents of the preferred embodiment at a preselected speed.Furthermore, the approximate value of the frequency of the output signalfrom optodetector 259 and the speed of the driven element, for example,photoreceptor belt 38, is known.

With this background, the digital electronic controller of the preferredembodiment will now be described in connection with FIGS. 11 and 12.Turning first to FIG. 11, a block diagram of the overall electroniccontroller of the preferred embodiment is shown. The entire operation ofthe copying machine of the disclosed embodiments is controlled by aditital controller board 20 shown in the middle of FIG. 11. Digitalcontroller 20 is in two-way communication with each of the other controlboards, upper driver board 275 and lower driver board 276. Upper driverboard 275 provides drive to the solenoids in toner modules 136, whichwill be described in greater detail hereinbelow. A plurality ofconventional paper path and paper tray quantity sensors are shown as 277and 278, respectively. Additionally, a transfer belt cleaning solenoidis shown at 279 which selectively moves a blade into and out of contactwith transfer belt 39 after a composite image has been transferred to animage receptor.

Lower driver board 276 controls paper path clutches 55. Additionally,this board drives paper drive motor 56 and fuser drive motor 65. Aplurality of conventional paper tray identity sensors 280 alsocommunicate with controller 20 through lower driver board 276.Additionally, machine interlocks 281, which detect whether all panelopenings and the like are properly closed, communicate through lowerdriver board 276. Also, a fuser temperature sensor 282 connects throughlower driver board 276.

As described in greater detail in connection with FIG. 12, the processoroperating digital controller 20 includes a plurality of analog inputlines and on-board analog to digital converters. The various analogvoltages sensed throughout the electrical system of the preferredembodiment are connected directly (through plug connectors to thevarious driver boards) to these analog inputs so that the digitalcontroller can respond to these signals.

A substantially conventional main power supply is shown at 285. However,controller board 20 communicates certain signals to main power supply285 to implement the novel energy management features for the copier ofthe preferred embodiment. Power supply 285 provides power to fuserheater 286, expose lamps 287, and an AC cooling fan 288, all of whichare controlled by triacs.

Controller board 20 also communicates with switches in display on frontpanel 85 and the high voltage supplies 289 for controlling the inputvoltages to the various coronas within the copier. Controller 20 alsocommunicates with electromechanical control devices in optics bench 290to synchronize the optical scan of an original to the movement ofphotoreceptor belt 38. Lastly, digital controller 20 also controlssensitizing coronas and interdocument control lamps at expose station291 and receives the output from electrostatic voltmeter 123 (FIGS. 5,6).

The architecture of the overall electronic control of the presentinvention is designed to distribute the work to be done by variouselectrical components in a manner which allows all of the necessary workdone by high-speed digital controller 20 to be accomplished in realtime, and to distribute other tasks which are less time critical toboards 275 and 276. Once the basic structure of digital controller 20 isdescribed, implementation of the other control functions present onboards 275 and 276 will be apparent to those skilled in the art in lightof the explanations contained in the specification of how thesefunctions interact with the controller. Additionally, a significantnumber of the control functions shown in FIG. 11 are conventional in theart of electrophotographic copiers and laser printers.

A partially block and partially schematic diagram of the digitalcontroller 20 of the preferred embodiment is shown in FIG. 12, whichconsists of FIGS. 12A and 12B. The digital controller 20 is constructedaround a high-speed microprocessor 310. In the preferred embodiment,processor 310 is implemented by a type 8095 high-speed microcomputercurrently manufactured by Intel Corporation of Santa Clara, Calif. Amaster source of clock and timing signals is provided by a 12 megahertzcrystal clock 21 which connects to an appropriate input pin of processor310 via line 313.

The 8095 is a high-speed 16-bit processor which includes a multiplexedaddress and data bus. The lower order 8-bits of this bus are shown at311 with the higher order 8-bits being sub-bus 312. Extensions of thesesub-buses are provided to a conventional 16-bit address latch 315 whichholds an address on its 16-bit output bus 316 in response to an addresslatch enable signal appearing at output 317 of the processor. Also, apair of conventional 8-bit bi-directional bus transceivers 318a and 318bcontrol data flow on the bus on its extensions appearing at 319 and 320.

The use of address latches such as latch 315 in response to addresslatch enable signals in a processor having a single bus whichmultiplexes address and data signals will be well known to those skilledin the art, and need not be described in detail. Extensions 319 and 320of the 16-bit bus are provided to a memory block which bears referencenumerals 22 and 26 to indicate that it contains both read only memory 22and random access memory 26 shown in FIG. 1. Naturally, the read onlymemory contains look-up tables and some of the instruction signals forcontrolling processor 310.

A plurality of conventional decoders and gates are all represented byaddress decoders and control block 321. Implementation of such devicesis well known to those skilled in the art. In particular, the I/O forcontroller 20 is memory mapped. Implementation of appropriate outputsfrom address decoders and control block 321 to provide signals shown onthe lines coming from this block are within matters well within theordinary level of skill in the art and thus a detailed circuit of thepreferred embodiment of these elements is not shown. It is noted thatthis block responds not only to 16-bit address bus extension 316 but toread and write signals from processor 310 which appear on lines 322 and325, respectively, and to the address latch enable signal which appearson line 326. A plurality of lines indicated as 327 select particularchips within the ROM 22 and RAM 26 in response to decoded addresses onbus 316 in a conventional manner.

Controller 20 communicates with other portions of the machinery of thepreferred embodiment via five primary pathways. The first is a serialdata link 328 which is provided to the keyboard and display on frontpanel 85 (FIG. 2).

Secondly, a collection of input signals shown as 329 on FIG. 12 areprovided to a type 8259 interrupt controller chip shown as 330. Inresponse to receipt of one or more signals on lines 329, interruptcontroller 330 provides an interrupt output on line 331 which isconnected to the external interrupt line 332 of processor 310.

The third main communication path is through four lines showncollectively as 335 which are connected to three high speed interruptinputs on the 8095. Each of the inputs on lines 335 are generated bylight choppers of the type shown in FIG. 10. Line 45 carries the outputof the light chopper 41 from the transfer belt motor. Line 44 carriesthe output of the light chopper connected to the photoconductor belt andline 336 carries the output from a light chopper on the scanner motorfrom the optics bench. Line 343 carries output from the light chopper onpaper drive motor 56 (FIG. 4A) Most of the communication functionsbetween controller 20 and driver boards 275 and 276 (FIG. 11) takesplace through a serial I/O block which is surrounded by dashed line 338in FIG. 12.

Lastly, a collection of timing signal outputs are shown collectively as339 on the right-hand side of FIG. 12. These outputs include pulse widthmodulated output signals for controlling the speeds of the five motorsused in the preferred embodiment, the 15 kilohertz signal for the tonersensor of FIG. 8, which appears on line 227, and other timing signals.

Processor 310 is very busy. As will be apparent from the foregoingdescription, and the additional description herebelow of the functionscontrolled by the processor, it will be appreciated that thecommunication scheme shown in FIG. 12 has been adopted in order tomaximize the amount of information that controller 20 can handle andgenerate, and to make sure that top priority routines get servicedfirst.

To this end, the four high speed interrupts of the 8095 which areconnected to lines 335 suit the environment of the present inventionparticularly well. As is known to those skilled in the art, the highspeed interrupts of the type 8095 have a somewhat special configuration.Whenever an interrupt signal is received at one of these pins, theprocessor immediately (within one clock cycle) latches in a value froman internal clock in the processor indicating the time of occurrence ofthe interrupt signal. The processor then proceeds to service theinterrupt in an otherwise conventional fashion. However, the informationabout the time of the occurrence of the interrupt remains latched and isused by the processor to precise calculations, since the time ofoccurrence of the interrupt is known to a high degree of precision.Therefore, the high speed interrupt inputs of this processor areparticularly useful in implementing the position servo for the transferbelt, the photoreceptor belt, and the light scanner motor as indicatedin FIG. 12.

It should be appreciated from the description of the preferredembodiment to this point that proper synchronization of thephotoreceptor belt, the transfer belt and the scanning motor in theoptics bench are the critical elements for proper synchronization of theimage creation and development components of the print engine of thepresent invention. Additionally, the paper drive motor must have itsspeed well regulated during transfer of an image from the transfer beltto a sheet of paper to prevent distortion of the resultant image.Therefore, in the preferred embodiment, the motion transducers, in theform of the light choppers described in connection with FIG. 10, havetheir outputs connected to these high speed interrupt pins so that theposition of these elements may be determined with great accuracy.

Less critical signals which are serviced by interrupt routines ofprocessor 310 are controlled by interrupt controller 330. In thepreferred embodiment, this device is implemented with a type 8259Ainterrupt controller. As is known to those skilled in the art, thisinterrupt controller will arbitrate the priority of the variousinterrupts connected to it and provide information to processor 310about these interrupts. As noted above, the receipt of at least oneinterrupt signal on one of the input terminals IR0-IR7 of controller 330causes an interrupt signal to be placed on line 331. An interruptacknowledge signal (not shown) is provided back from address decoder 321to interrupt controller 330 in a manner known to those skilled in theart.

In response to the interrupt acknowledge signal, controller 330 placesan 8-bit word on its data output lines shown as 340. These lines aredirectly connected to low order bus extension 320. Processor 310appropriately disconnects other devices from the bus, and reads the dataon lines 340 through bus transceiver 318b to determine the interrupt tobe serviced. The processor then proceeds to service the interruptroutine. Note that the order of priority for the interrupt signalsconnected to controller 330 include the mark sense and the home sensesignals from the optics bench.

The home sense signal on the optics bench is generated by a sensor whichdetects the optical scanner returning to a home position. This is usedto indicate to digital controller 20 that the optics bench is preparedto start another scan. The mark sense signal is generated by a sensorwhich is located at a position along the path of the optical scannerwhich just precedes the point at which the area of the photoreceptorbelt which will carry images actually to be transferred onto paper isilluminated by the scanner. This signals the digital controller thatscan of an original is about to begin, and alerts the controller to thefact that the optical scanner should have ramped up to a constant speedat the time the mark sense signal is generated.

Next, in priority is a signal on line 50 indicating that that thephotoconductor index mark has been detected. Similarly, the signal online 51 is generated when the transfer belt index mark is detected.Lastly, a line sense signal for detecting zero crossings of the 60 Hz ACline signal (for use with the triac controls) and light chopper outputfrom the fuser drive motor, appear on lines 345 and 346, respectively.

The lowest priority interrupt signals to the controller are generated bythe receive data ready and the transmit data ready outputs on lines 347and 348 from a type 8251A UART 349. UART 349 communicates 8-bit parallelwords to processor 310 via an extension of the lower order bus 320.Also, read and write signals are provided on lines 350 and 351 in aconventional manner to control the bi-directional flow of data throughthe UART. As noted above, a serial link 328 is implemented by thetransmit data 31, receive data 30, and data terminal ready (DTR) linesof UART 349. In the preferred embodiment, UART 349 is used to implementan RS-232 port so that other devices may be connected to the printengine.

As will be appreciated by those skilled in the art, serial link 328 is arelatively low traffic communications path since it must only respond tointermittent operation of keys from the keyboard and provide appropriateinformation to the optics bench about what elements of display 87 (FIG.2) should be activated to inform the user of the operating status of themachine. The adoption of a serial data link minimizes the number ofconnections necessary between the print engine of the preferredembodiment and the optics bench. Similarly, relatively low speed datafrom a laser bench used in constructing an embodiment of the preferredinvention may also be communicated through UART 349.

The main timing output signals from controller 20 are shown collectivelyat 339 in FIG. 12B. These are generated by a pair of type 8254programmable interval counter currently manufactured by IntelCorporation of Santa Clara, Calif. These devices are programmed throughdata provided on bus 320 in response to chip select input signalsprovided on lines 355 and 356. Additionally, the two lowest orderaddress bits from bus 316, which are shown as 357, are used to selectparticular internal registers of the 8254s when the address of one ofthem has been decoded by address decoder 321. As noted hereinabove,these devices provide pulse width modulated output signals forcontrolling the DC motors in the preferred embodiment.

In addition to those already described, the pulse width modulated signalfor the scan motor appears on line 358, that for the fuser motor on line359, and that for the paper drive motor on line 360. Other timingsignals provided by programmable interval timers 353 and 354 arerepresented generally at 361 and 362.

Lastly, serial data is received and transmitted by controller 20 throughthe bi-directional serial I/O pin 365 from processor 310 labeled RXD.Clocking information for this serial data is provided in response tooutput on TXD pin 366.

A 4-bit select word is written into register 367 which is embodied by atype 74273 octal D flip-flop circuit. When processor 310 is writing aselect word to this device, the device's address is detected by decoderand controller block 321 and a clock signal is provided on line 367 tolatch the control word into register 367. When outgoing data isprovided, a particular 3-bit word is written onto 3-bit output selectbus 368. This bus is connected to two type 74138 3-line to one of eightdecoders 369 and 370. The select inputs to decoder 369 are controlled byan RXD output 365 which appears on line 371. The select inputs ofdecoder 370 are controlled by TXD output 366 which appears on line 372.

Therefore, when processor 310 needs to provide serial data out throughpins 365 and 366, it first writes a select word into register 367 whichselects the particular one of data output lines 375 and clock outputlines 376 which will be activated. It should be understood that theseare associated on a one-to-one basis, and therefore when data to aparticular one of the other boards or modules of the preferredembodiment is to be provided on one of the data lines, a correspondingclock line is provided to the same device.

An additional 74138 decoder (not shown) is connected to an output oflatch 367 to provide strobe signals to registers on the other boardsconnected to lines 375 and 376. These signals latch data from shiftregisters which receive the serial data.

Only four outputs for each of decoders 369 and 370 are shown. Otherdecoded outputs include a strobe signal provided to other parts of thepreferred embodiment requesting that internal data be provided to theprocessor about the states of a plurality of switches which indicatewhether the various hinged panels and other elements of the machine areproperly closed, various switches which determine the level and size ofpaper in the paper trays, and so forth. When this line has beenactivated, driver boards 275 and 276 (FIG. 11), through UARTs located onthose boards, provide serial data back to serial communications block338 via internal data line 377.

Line 377 is connected to the three input of a type 74251 eight to onemultiplexer 378. When the processor determines that this data is to beread, an appropriate select word is written into register 366 whichplaces a 011 state on select bus 368. This selects the number 3 input,and thus line 377, to be provided to the output of the multiplexer 378on line 379. Also, the select word written into register 366 provides anappropriate signal on line 380, which is now provided as a part of otherselect words, to take the multiplexer output on line 379 from its highimpedance state to its active state. Therefore, under these conditions,data coming in on line 377 appears as output on line 379 and is in turnprovided over line 371 as input to RXD pin 365 of processor 310.

Analog voltages from various transducers in the preferred embodiment areprovided along lines 380 and 241 to the inputs of a triple two-to-onetype 4053B analog multiplexer 374. The select inputs of this multiplexerare controlled by lines 373 which are connected to the outputs of latch363. The three output lines 381 from multiplexer 374 are provided tothree analog inputs of the type 8095 processor. As should be known tothose skilled in the art, this processor includes a plurality of analoginput pins with on-board analog to digital conversion apparatus forgenerating digital words proportional to the analog input voltages. Whenprocessor 310 determines that one or more of these voltages is to beconverted, an appropriate select word is written onto lower order busextension 320. Address decoder and controller 321 provides a strobeoutput on line 364 which latches the select word onto the outputs oflatch 363, thus selecting particular ones of lines 241 and 380 to beprovided to the processor's analog inputs.

The response of controller to synchronize belts 38, 39 in the preferredembodiment will now be described. It should be understood that as a partof this description, appropriate control signals are also provided tosynchronize operation of the scanning motor on the optics bench.However, the essence of the present invention is the print engine itselfwhich, as noted above, may be used with either an optics bench, a laserbench, or another selectively actuable light source which may be usedfor writing images onto a photoreceptor.

Digital controller 20 implements a digital servo for each of the motorsconnected to the transfer belt, the photoconductor belt, and the scannermotor. In each of these, information about the current mode or position,and therefore the current position of the device driven by the motor, isprovided through the above-described light choppers. The photoconductorbelt is nominally thirty-eight inches long with a tolerance of plus orminus 50/1000ths of an inch. Transfer belt is nominally nineteen incheslong with a similar tolerance of plus or minus 50/1000ths of an inch.The servo implemented by digital controller 20 regulates belt-to-beltregistration within 1.5 mil, or 0.0015 inch. This provides a resultantimage on the paper wherein each position on one of the composite imagesis assured of being at its proper location within a tolerance of5/1000ths of an inch.

In order to implement this, processor 310, in connection with the othercircuitry of controller 20, implements the following functions. First,processor 310 generates a precision 500 kiloHertz internal clock that isused in connection with the servo.

It should be understood that teeth 258 on light chopper wheels 256,together with the gearing arrangements between the motors driven and therollers carrying belts 38 and 39 are spaced such that a passage from apoint on one tooth to a point on the next adjacent tooth corresponds toapproximately three mils of linear movement of one of the belts.Therefore, something slightly more than 12,600 outputs fromphotoreceptor belt light chopper 41 are provided during one completerevolution of the 38-inch belt. Approximately half this number areprovided when the transfer belt makes one complete revolution.

However, due to the above-referenced mechanical imprecision of the beltsthemselves, the gears within the motor assemblies, and thecompressibility of rubber contained on rollers driving the belts, theexact correlation between movement from a position on a tooth to aposition on next adjacent tooth to movement of a particular length ofbelt cannot be assumed to be a constant from machine to machine, nor canit be assumed to be a constant as the machine operates over a period oftime. Therefore, an outer loop to the servo system is executed by acontroller 320. It should be noted that in this specification, thisservo will be described in connection with photoconductor belt 38.However, a similar program is executed for both transfer belt 39 and themotor on the scanning light of the optics bench.

On every revolution of the photoconductor belt (every other revolutionof the transfer belt) a count is maintained of the number of interruptsgenerated by the associated light chopper. When index mark 251 isdetected by optodetector 252 (FIG. 9), this length count is stored andcorresponds to the total length of the belt. During execution of thespeed control servo, this count is used as representing the total lengthof the belt. Therefore, it should be apparent that controller 20acquires a length for the belt expressed in terms of the number ofoutputs from its associated light chopper.

At this point, it should be noted that the time value stored in responseto an interrupt on one of the high-speed interrupt lines 335 is preciseenough so that a rough interpolation of a fractional portion of a toothon the light chopper can be made when the photoconductor belt index markis detected, as indicated by an interrupt signal on line 255.

In the preferred embodiment, movement of the photoconductor belt isconsidered the master, and movement of the transfer belt and the lightscanner motor are slaved to the photoconductor belt.

The preferred embodiment of the present invention also executes what maybe considered an innerloop of a digital servo system. FIG. 13 is a blockdiagram representing the serve loop wherein digital components basicallyimplement well known analog components of a serve system suggested bythe loop structure. It should be understood that in one sense, FIG. 13may be considered a rudimentary flow chart for the program running inprocessor 310 (FIG. 12A) to control the motors.

A precision clock 385 is a 500 kiloHertz clock signal internal toprocessor 310 which is derived from crystal clock 21 (FIG. 12A). Block386 represents generation of an interrupt every three milliseconds whichis the sampling frequency of the digital servo. Both of these devicesare shown as being connected to a 32-bit register 387 internal toprocessor 310. During execution of the above-referenced outer loop, thecontroller acquires a count value for the number of interrupts generatedon lines 44, 45, and 336 (FIG. 12), for a complete rotation of the beltsand scan of the optical scanner. The lengths of these devices are known.Therefore, in order to synchronize the speed of movement of thesedevices, the digital controller calculates a ticks per unit timeequivalent for generating the desired speed of approximately ten inchesper second. As used in this statement, a tick refers to a transition onone of lines 335 indicating that a complete cycle from one tooth to anadjacent tooth has been experienced on one of the light chopper wheels.

This result is used to calculate a target position for the belt eachthree milleseconds. As noted above, this value was loaded into 32-bitregister 387. The most significant 16-bits of this register representthe integer number of ticks at the target position and the leastsignificant sixteen bits represent an interpolated fraction of the tick.In practice, only about the most significant four to six bits of thisfractional part are used. Block 388 represents a sampling gateindicating that a calculated target position shown on line 389 isprovided to adder 390 during each sampling period.

Motor 36 is driving light chopper 40, and this provides a count to acounter in processor 310 represented by counter 391 on FIG. 13. Itshould be understood that counter 391, as described hereinabove,includes an upper sixteen bits which represent the actual integer numberof counts received from light chopper 40 as well as an interpolatedfractional part calculated from the stored time of occurrence from themost recent high-speed interrupt generated to the 8095 processor. Theoutput of this counter represents a current belt position and isprovided to digital adder 390 on line 392. Naturally, element 390 onFIG. 13 represents a subtraction step performed by processor 310.

As is known to those skilled in the art, the output of adder 390 is anerror signal appearing on line 395. This is provided to a digital filter396 which is also executed in software by processor 310. Digitalfiltering is provided for stability in a manner directly analogous tothe use of an analog loop filter and an analog servo. Therefore, adigital error signal, shown at 397, is the filtered output from filter396 which is provided as an input on line 398 to a control input forpulse width modulator 353. The pulse width modulator is given the samereference number as the device which implements it, programmableinterval timer 353 shown on FIG. 12.

A reset block is shown on FIG. 13 as 399. It should be understood thatin a loop controlling the photoconductor belt, reset occurs each timethe home position of the belt is detected by an interrupt beinggenerated on line 50 (FIG. 12). The other servo control devices whichare slaved to the photoconductor belt receive a reset based on detectionof the home position of the photoconductor belt. Therefore, if thetransfer belt has lagged slightly behind the photoconductor belt duringits most recent revolutions (one revolution of the photoconductor belt),the target position register for the loop controlling the transfer beltis forced to a zero value when zeros are placed in the register of theloop controlling the photoreceptor belt. As will be apparent to thoseskilled in the art of control systems, this will cause the transfer beltto somewhat abruptly speed up or slow down to try to reach its newtarget position, assuming that it has either fallen behind or gottenahead of the photoconductor belt.

As noted above, during image transfer between the two belts, very strongelectrostatic forces exist between the two. Therefore, theabove-referenced seam areas of each belt are selected as the contactpoints on the belt wrap, discussed in connection with FIGS. 5 and 6, forthe resynchronization of the belts. The corona 198 (FIG. 6) associatedwith the transfer belt is used to apply an appropriate charge conditionto the seam area of the transfer belt. Therefore, when the two seamareas of the belts come in contact, the above-referenced strongelectrostatic forces are not present and the transfer belt servo canadjust the position of the transfer belt relative to the photoconductorbelt.

From the foregoing it will be appreciated that the use of these servomechanisms is what maintains the proper registration for the compositeimages recited above. Naturally, in embodiments of the present inventionin which the belts are of equal length, such an adjustment would occurfor each revolution of the transfer belt rather than every otherrevolution.

The inventors of the present invention also experimented with a servosystem which compensated for slight deviation from the integersub-multiple relationship with the belt lengths by operating the beltsat different speeds, to synchronize the arrival of the seam areas at thebelt wrap position. This system was designed to assure that properregistration is maintained near the top of a composite image and thatany improper registration resulted in a uniform slight smear as oneproceeded down the image along the length of the belt.

While it is believed that embodiments of the present invention may beconstructed using this form of control, it is less preferred. Inparticular, when the transfer belt was moving at a speed slightly lessthan that of the photoconductor belt, the strong electrostatic forcesbetween the belts from the charged areas were in contact and tended toslow down the photoconductor belt which in turn produced a high errorsignal trying to cause it to speed up. This was found to create a lackof stability in the system.

Turning next to FIG. 14, a diagram of the improved fuser of the presentinvention is shown. The fuser mechanism is driven by a common chain 410.The tension adjustment gear for the chain is shown at 411. Shaft 412 isfrom the reduction gears connected to motor 65 (not shown in FIG. 14).Gear 415 drives a top heated roller and gears 416 and 417 drive a pairof rollers 418 and 419, respectively. A fuser heater element is shown as420 within top roller 421. Heat from roller 421 is transferred tocompressible heated roller 422 within the interior of the fusermecahnism which may be seen because a portion of the outer view of thefuser mechanism in FIG. 14 is shown as broken away at line 425. Roller422 has a rubberized compressible surface 426 which is indirectly heatedby roller 421.

An oil pad 413 is periodically lifted to contact roller 414 to providesilicone oil. This oil is transferred via roller 423 to the otherrollers within the fuser.

As noted hereinabove, the improved fuser of the present inventionprovides increased dwell time of the image receptor or paper within thefuser. This is accomplished by using a pair of rollers 418 and 419 whereprior art fusers have uniformly used a single roller. The outer surface424 of first compression roller 418 is covered with material having agreater compressibility than that of surface 426 of heated roller 422.This prevents paper from tending to be peeled away from heated roller422 as it proceeds from its entrance point to the fuser at 427 to itsexit point 428. Plastic deflector 429 is included within the fuser tocatch a leading edge of a sheet of paper which may tend to becomeseparated from heated roller 422 and guide the sheet onto the junctionbetween compression roller 419 and heated roller 422. From the foregoingit will be appreciated that the paper is in contact with heated roller422 for the entire arc length of an angle indicated as A in FIG. 14.This provides a much greater dwell time for the copy within the fuserthan that achieved by the prior art fusers in which the dwell time waslimited to the time the sheet of paper was compressed between a heatedroller such as 422 and a compression roller such as either roller 418 or419.

FIG. 14A is a detail drawing showing the paper contact area amongrollers 418, 419 and 422 of the fuser shown in FIG. 14. As noted above,the relative compressibilities of the surfaces of these rollers is animportant design feature of the improved fuser of the present inventionin order to prevent paper from becoming separated from surface 426 ofroller 422 as it travels between rollers 418 and 419. In ascending orderof compressibility of their surfaces, the rollers are 419, 422, followedby 418. Roller 418 is constructed of foam rubber in the preferredembodiment, with its outer coating 426 being formed of a smooth rubber.Heated roller 422 has an outer coating of a harder rubber.

Sheet-like material passing between heated rollers of differentcompressibilities will tend to cling to the contour of the roller whichis less compressible, and thus has an outer surface which most closelyapproximates retention of its arcuate shape. Therefore, when paperenters the area at 427 between surfaces 426 and 424, it tends to clingto surface 426 as roller 422 rotates clockwise, since surface 426 isharder and less compressed than surface 424.

After passing deflection finger 429, the edge of the paper enters thearea, shown at 430, between surface 426 of roller 422 and surface 431 ofroller 419. In the preferred embodiment, roller 419 is constructed ofaluminum with an outer surface 431 consisting of a coating of Teflon.Since the aluminum and Teflon roller 419 is harder than the rubberizedsurface 426 of roller 422, surface 426 is deformed more greatly thansurface 431. This causes the paper to tend to follow surface 431 ofroller 419 as it exits the fuser mechanism and therefore to peel awayfrom heated roller 422 so that it does not jam within the fuser.Therefore, the improved fuser of the present invention meets theabove-stated object of the invention of providing increased dwell timeat a given speed of operation.

Additionally, the digital controller 20 shown in FIG. 12, receives asignal over internal data link 377 when a copying sequence is about tobegin. The fuser of FIG. 14 is equipped with conventional temperaturedetectors for monitoring the temperature of the fuser. Heater element420 is activated under the control of digital controller 20 (FIG. 1)when the temperature becomes too low. Whenever the fuser heater is on, ascanner enable signal from digital controller 20 is placed in acondition in which it will prevent operation of the lamps and preventthe beginning of an optical scan until the fuser has reached anappropriate temperature. Similarly, when a scan is in progress, digitalcontroller 20 will not turn on heater element 420 to elevate the fusertemperature until the scan is completed. Since the fuser heater element420 and the two above-referenced 400 watt lamps used in the opticalbench are the devices which draw the most current in the preferredembodiment, it will be appreciated that the preferred embodiment can beoperated safely and dependably off a conventional 15-ampere 120-voltbranch circuit without overloading same.

Two embodiments of the improved toner modules 136 are shown in FIGS.15-18. FIG. 15 shows a first embodiment of the improved downwardlyfacing gravity fed toner module. FIGS. 15-18 are cross-sectional viewsof what one would see when facing the modules in the views of FIGS. 5and 6.

A hopper 440 holds a supply of plastic toner materials 441. The bottomof the hopper has downwardly sloped edges 442 which guide the tonerparticles. A ratcheting toothed shaft 445 is indexed each time thedigital controller detects a condition of low toner in a mixing chamber446. A plurality of teeth 447 around the surface of shaft 445 introducea predetermined amount of toner material into the mixing chamber eachtime the shaft is indexed.

A pair of mixing augers 448 and 449 move the mixed combination of tonermaterial and ferromagnetic carrier longitudinally within the mixingchamber. The pitch and direction of rotation of augers 448 and 449 areselected so that one moves material into the page and one moves materialout of the page in the view of FIG. 15.

A decorator roller of substantially conventional construction is shownat 450. This roller includes a rotating aluminum sleeve 451 which isdriven about a stationary magnetic core 452 having a non-magnetic innersection 455. One of the novel aspects of the toner deposition apparatusof FIG. 15 is the inclusion of mixing roller 456. Mixing roller 456 alsohas a rotating outer aluminum sleeve 457 and a magnetic core 458 with anon-magnetized inner section 459. Each of rollers 450 and 456 rotatecounterclockwise as seen in FIG. 15. Therefore, the left-hand sides ofthese rollers, as viewed in the figures, are downwardly traveling andthe right side of these rollers are upwardly traveling. Within themixing chamber, the smaller stippling dots represent toner material 441and the larger dots, such as 460 show ferromagnetic carrier particles.

The pole orientations for magnetic cores 458 and 452 are shown in FIG.17. As is known to those skilled in the art, decorator rollers such asroller 450 are arranged to have a relatively smooth outer sleeve,usually constructed of aluminum, rotating around a magnetized corehaving radially spaced magnetic poles of differing polarity. Because ofthe magnetic field generated by the core, the carrier particles tend tocling to the outer sleeve. As rotation of the outer sleeve carries themixture over a portion of the magnetic core which lies between two polesof opposite polarity, such as position 465 shown in FIG. 17, the fluxlines of the magnetic field tend to be tangential to the outer surfaceof the core, and thus to the surface of the aluminum roller. This causesthe mixture to tend to lay down.

As the particles move from this portion of the magnetic field to aposition where they are at the center point of a pole of particularpolarity, such as point 466 shown in FIG. 17, the particles tend tobecome aligned with the field, and thus form a brush which will standaway from the surface of the decorator roll and contact photoreceptorbelt 38. This phenomenon is illustrated in FIG. 15 at correspondinglocations 465 and 466 on the outer surface of decorator roller 450.

As is known to those skilled in the art, this phenomenon occurs becausethe ferromagnetic carrier particles are normally chosen to be somewhatelongated. Therefore, in the presence of a magnetic field, inducedmagnetic dipoles form within these particles along their longestdimension. Therefore, the elongated particles tend to stand up when inthe presence of a magnetic field for which the magnetic flux or fieldintensity vector is pointing radially from the center of the decoratorroller, and tend to lay down next to the roller when the magnetic fluxvector has a strong tangential component. In the preferred embodiment,the pole pieces used provide a magnetic field intensity of approximately6000 gauss.

As may be seen in FIG. 15, the brush of particles contacts photoreceptorbelt 38 at location 466. The portions of the surface of belt 38 carryinga high positive electrostatic potential will draw the particles on tothem in a conventional manner.

As particles which remain on the surface of aluminum sleeve 451 arecarried upward, they next pass a centerline 467 of a north pole withincore 452. This corresponds to location 467 shown in FIGS. 15 and 17.Therefore, another brush of particles is formed within the interior ofmodule 136. As described hereinabove, the radial component of themagnetic field is at its strongest at location 467, as it is at location466, and the brush formed helps keep particles from falling back underdecorator roller 450.

As the particles continue to proceed around the upwardly traveling edgeof sleeve 450, they approach point 469 (FIG. 17) between the north polesection 469 and an unmagnetized portion of the core 470. This portion ofthe surface of decorator roller 450 lays under the center of a southpole segment 471 of mixing roller 458, the center of which is shown as472 in FIG. 17. Therefore, the particles ascending on the right side ofdecorator roller 450 enter an area of very weak magnetic field from core452, and are pulled by the strong field of south pole segment 471 ofcore 458 so that they become magnetically adhered to rotating aluminumsleeve 457 of mixing roller 456. The particles are thereby lifted upinto the mixing chamber and further agitated, by alternately laying downand standing up, as they pass over magnetic pole pieces 475-477 shown inFIG. 17.

Auger 448, is positioned with respect to aluminum sleeve 457, so that itpulls a significant portion of the particles away from surface 457 formixing in mixing chamber 446, as described hereinabove. Note that auger448 is positioned so that it is approximately centered over north polesegment 477 and thus the particles will tend to be standing when theypass under the auger. Those particles not pushed along by auger 448 willcontinue on to the downward traveling left-hand side of mixer roller456. As may be seen in FIG. 17, they then pass over a non-magnetizedportion 478 of core 458. Thus, in response to gravity, some of theparticles fall downward onto the surface of aluminum sleeve 451.

In the first preferred embodiment of FIG. 15, a selectively rotatabledoctor blade 479 is employed. The doctor blade, as shown in FIG. 15, isin the open position which allows particles to pass to and form thebrush at 466, described hereinabove. When the doctor blade is rotated toits closed position, shown in phantom in FIG. 15, it scrapes particlesaway from the surface of aluminum sleeve 451 and they tend to collect ina pile, shown in phantom at 480 in FIG. 15. Doctor blade 479 isselectively rotated between two stopped positions by the use of asolenoid in the preferred embodiment.

The extent of angular rotation of doctor blade 479 in the open positionis selected so that it is approximately three millimeters away from thesurface of aluminum sleeve 451 when in the open position. This allows anadequate thickness of particles to adhere to the surface of thedecorator roller and pass down to form a brush at 466, without droppingthe accumulated pile 480 onto photoreceptor belt 438.

As noted hereinabove, the use of a downward facing gravity-fed tonermodule provides a system which is less susceptible to contamination whena condition of carrier pull from another module exists within themachine. If it is assumed that an adjacent toner module is active, and acondition of very high charge on the surface of belt 38 has causedcarrier pull to occur from the other module, doctor blade 478 will be inits closed position and therefore the portion of surface 451 at point466 will be free of particles. As the pile of particles, which includescarrier particles and toner which are not strongly electrostaticallyattached to belt 38 passes under the module of FIG. 15, the only waycontamination can occur is for sufficient magnetic attraction to existbetween the carrier particles laying on the surface of the belt anddecorator roller 450. In conventional systems, particles laying on thesurface of a belt after a carrier pull tend to be shaken off and to fallinto other toner modules which are facing upward, thus contaminating thecolor of the toner.

The inventors of the present invention have found that the downwardfacing module does not cause the expected problems of toner falling ontothe surface of photoreceptor belt 38 when toner deposition is notdesired.

Furthermore, the use of the novel mixing roller 456 is advantageous inthat it requires no augers for lifting toner and carrier particles ontothe surface of a decorator roller as is done in prior art color copyingsystems. Mixing augers 448 and 449 are the only augers used within thesetoner modules.

A second embodiment of an improved downwardly facing gravity-fed tonermodule is shown in FIG. 16. In FIG. 16, all like parts to the module ofFIG. 15 are referenced with the same numerals. The mixing roller isshown as 456' to indicate that it serves substantially the samefunction, but has a different structure, than mixing roller 456 of theembodiment of FIG. 15.

Similarly, decorator roller 450' of the embodiment of FIG. 16 has adifferent internal structure than decorator roller 450 of the embodimentof FIG. 15. Aluminum sleeves 451 and 457 are of the same type as thosebearing the same reference numerals in the embodiment of FIG. 15.

The magnetic pole orientation of magnetic cores 482 and 488 in FIG. 16are shown in FIGS. 18A and 18B. In the embodiment of FIG. 16, magneticcore 482 of the decorator roller includes a non-magnetized inner section455' to which it is securely fixed. Passing to the center of themagnetic core is a shaft 485 which may be rotated between two stoppedpositions by a solenoid (not shown) to rotate the magnetic core.

The magnetic pole orientation of core 488 is shown in FIGS. 18A and 18B.It includes a downwardly facing 80° south pole segment 486 which isapproximately centered at 487. As one proceeds in the direction ofrotation of sleeve 457 (counterclockwise), alternate north and southpoles are encountered until non-magnetized portion 489 is reached.

The combination of the selection of magnetic pole orientation for core482 and the angle through which it is rotated between the open andclosed portions shown in FIGS. 18A and 18B, respectively, form amagnetic gate for toner and carrier particles in the embodiment of FIG.16. First, consider the orientation of core 482 in the open positionshown in FIG. 18A. In this configuration, as particles reach the lowerportion of the unmagnetized section 489 of core 488, they tend to bedrawn off the surface of sleeve 457 under the influence of gravity andmagnetic dipole formed between south pole segment 486 of core 488 andnorth pole segment 490 of core 482. The particles then move downwardlyover unmagnetized segment 493 on aluminum sleeve 451, over north polesegment 491 and south pole segment 492. A brush is formed at 466 as wasthe case with the embodiment of FIG. 15 when the magnetic gate formed bycore 482 is in its open position.

Next, assume that this particular toner module is no longer active andthe gate closes. This is the condition represented in FIG. 16 and shownin FIG. 18B. Contrasting FIGS. 18A and 18B, it may be seen that magneticcore 482 has been rotated approximately 45° counterclockwise. When thisoccurs, the magnetic gate is shut, as may be appreciated from viewingFIG. 18B and FIG. 16 which shows the gate in a closed configuration.

In the closed position, unmagnetized section 493 of core 482 is movedunder position 487, which is the approximate center of a south polesegment of core 488. Therefore, no strong flux vector is aligned betweenthe lower descending side of sleeve 457 and core 482. The strongestlocal magnetic field which remains is the influence of south polesegment 486 and thus the particles remain adhered to the surface ofaluminum sleeve 457. They tend to stand up and form a brush as they passover position 487 since this is near the center of south pole segment486. However, particles standing up at this position are directly over,and possibly in contact with, unmagnetized segment 493 of core 482, andtherefore the particles remain adhered to the surface of aluminum sleeve457.

Also, as particles are lifted on the rising side of sleeve 451 of thedecorator roller, they pass over north pole segment 490 and onto an areaover unmagnetized core segment 493 which lies directly under south polesegment 486 of the upper core 488. Therefore, the particles are drawnoff the surface of sleeve 451 and onto the surface of sleeve 457. Thus,as particles are raised by the right-hand ascending side of sleeve 451,immediately after the toner module is deactivated, they are transferredto the surface of mixer roller 456' when the gate is shut.

It is believed by the inventors of the present invention that evenadditional advantages may be obtained by constructing embodiments of thepresent invention in which both cores 482 and 488 are rotated when thegate is in the closed position.

In addition to magnetically closing a gate, so that particles remainadhered to mixer roller 456', the embodiment of FIG. 16 provides theadditional benefit of increased immunity from contamination. Consideragain the example of a condition of carrier pull from an upstream tonermodule. From inspection of FIG. 18B, it will be appreciated that atposition 466', the position lying closest to photoreceptor belt 38, themagnetic field orientation is almost purely tangential. Therefore, thereis very little tendency to pull any carrier particles laying on belt 38onto the surface of the decorator roller.

The preferred embodiment of the present invention uses three distinctimplementations of the toner modules of the preferred embodiment. Thefirst is that shown in FIG. 15 which is designed to be entirelydisposable. Sufficient carrier particles are retained within mixingchamber 466 to make approximately 7,000 copies. Similarly, hopper 440contains sufficient toner materials to make the 7,000 copies. When thissupply is exhausted, the entire module is removed from the machine,thrown away, and replaced with a similar module. Therefore, it ispreferred to use the embodiment having doctor blade 479 for thecompletely disposable module because same is easier and cheaper tofabricate than the rotating magnetic core 482 of the embodiment of FIG.16.

FIG. 16 represents a second embodiment in which the decorator roller isembodied as a permanent part of the print engine, and the section abovea joint shown at 495 and 496 is disposable. The preferred form of thisarrangement is to include within the mixing chamber sufficient carrierparticles to make 21,000 copies, and a similarly proportioned amount oftoner material 441.

An alternate arrangement has also been designed in which additionalremovable joints (not shown) are provided at 497 and 498 so that thehoppers 440 carrying toner materials are user replaceable anddisposable. The preferred form of this arrangement is to have the middlesegment of the toner module containing the mixing chamber and mixingroller 456 or 456' be removable, but not necessarily user replaceable.In this arrangement, sufficient carrier particles for 21,000 copies areincluded within the mixing chamber and individual hoppers 440, which areuser replaceable, are supplied with sufficient toner for 7,000 copies.Therefore, the user will be able to make three replacements of the tonersupply hopper and 21,000 copies before requiring service to clean themixing chamber and replace the carrier particles.

From the foregoing description of the preferred embodiment, and severalalternative embodiments, it will be appreciated that the presentinvention overcomes the drawbacks of the prior art and meets the objectsof the invention cited hereinabove. In view of the teachings of thisspecification, other alternative embodiments will suggest themselves tothose skilled in the art and therefore the scope of the presentinvention is to be limited only by the claims below.

We claim:
 1. A color print engine for use in an electrophotographicsystem comprising in combination:a first closed belt carrying saidphotoreceptor thereon having a characteristic first length, said firstbelt being driven around a pair of rollers laterally disposed so thatsaid first belt includes an elongated horizontal upper surface; a secondelectrostatic transfer closed belt having a characteristic secondlength, said second length being nominally an integer submultiple ofsaid first length, said second electrostatic transfer closed belt beingdriven about a plurality of rollers so that a predetermined portion ofsaid second belt contacts said first belt at one of said pair ofrollers; image source means disposed above said first belt and forcreating an electrostatic image on said photoreceptor in response tosaid light; at least four developer modules for carrying at least fourdistinct toners disposed above said elongated horizontal upper surfaceof said first belt, each of said developer modules including a tonerexit opening, and all of said toner exit openings being disposed in acommon plane substantially parallel to said elongated horizontal uppersurface of said first belt; a fuser disposed below said elongatedhorizontal upper surface of said first belt and to one side of thebottommost one of said plurality of rollers carrying said second belt; apaper tray disposed below said first belt and aligned with said fuser soas to provide a substantially straight horizontal paper path from saidpaper tray under said bottommost one of said plurality of rollerscarrying said second belt, on to said fuser.